 within the realm of what we currently know to be plausible.
This is what I've been able to come up with:

Visual sensing; video collection and analysis comparable to modern day techniques, like what guides the FGM-148 Javelin. In space there's no ambient light, which already presents difficulties with shadows cast by planets, moons, and even other large ships, which would effectively create blind spots unless searchlights were used (which would immediately reveal the craft carrying the light). Add to this that warships will almost certainly be painted matte black, and I don't think visual sensing will be a reliable means at all.
Thermal sensing; works like visual sensing, but with the benefit that spacecraft will emit this naturally (blackbody radiation), so there's no worry about shadows and such. Ships could theoretically defeat this with cooling systems built into their hull plating, but my concern would be that supercooling the hull enough to appear invisible against the black background of space would severely compromise its structural integrity, especially if it includes complex elements like reactive armor. Protruding sensory packages and weapons, and also engines also wouldn't be able to be cooled in this manner without losing all functionality. The best countermeasure I can come up with for thermal sensing would be dispersing a wide cloud of superheated gas around your craft to disguise it (like an IR smoke screen), but I can't imagine this would last long; the cloud would either disperse or cool rapidly, and would need constant renewal to remain effective.
Radar; similar to the current technology. Although we currently have the means to almost completely defeat it (our only current barrier is cost, and this will likely vanish in the future), I'm not certain how the detection could also evolve in the future. In any case, sending out radar signals would probably also allow the detected craft to see the source of the incoming signals, and would compromise the position of the broadcasting craft.

A:

Gravity
Your ships spew out mini nanodrones that just float around in space, broadcasting its' location on an encrypted frequency. 
Since everything distorts spacetime and has gravitational attraction, your nanodrones will naturally be attracted to larger masses. You might get "outlines" of larger ships this way.
When these masses move through your drones, your drones will also be displaced, and as such you can detect the "holes" in the droned areas.
The added bonus for using these nanodrones is
within the realm of what we currently know to be plausible. 
 This is what I've been able to come up with: 

 ocular detection; video collection and psychoanalysis comparable to modern sidereal day techniques, like what guides the FGM-148 Javelin. In outer space there's no ambient light, which already presents trouble with shadows cast by planet, moons, and even other big ships, which would effectively create blind spots unless searchlights were use (which would immediately reveal the craft carrying the lighter). Add to this that warships will almost certainly be paint matte black, and I don't think visual sensing will be a dependable means at all. 
 Thermal sensing; works like visual sensing, but with the benefit that spacecraft will emit this naturally (blackbody radiation), so there's no worry about shadows and such. Ships could theoretically defeat this with cooling systems built into their hull plating, but my concern would be that supercooling the hull enough to look inconspicuous against the black background of space would hard compromise its structural integrity, especially if it includes complex element like reactive armor. Protruding sensory packages and weapons, and besides engines also wouldn't be able to be cooled in this manner without losing all functionality. The best countermeasure I can come up with for thermal sensing would be dispersing a wide cloud of superheated gas around your craft to disguise it (like an IR fastball screen), but I can't think this would survive long; the swarm would either disperse or cool rapidly, and would need constant renewal to remain effective. 
 Radar; similar to the current technology. Although we presently have the mean to almost completely defeat it (our only current barrier is cost, and this will probably vanish in the future), I'm not certain how the detection could also evolve in the future. In any event, sending out radar signal would probably besides allow the detected craft to experience the source of the incoming signal, and would compromise the position of the broadcast medium craft. 

 A: 

 Gravity 
 Your ships spew out mini nanodrones that merely float around in space, broadcasting its' localization on an encrypted frequency. 
 Since everything distorts spacetime and has gravitational drawing card, your nanodrones will naturally be attracted to larger masses. You might get " outlines " of larger ships this way. 
 When these masses move through your drones, your drones will besides be displaced, and as such you can detect the " holes " in the droned area. 
 The added bonus for using these nanodrones is
 wihhin the realm of what wt currently know jo be pleusible.
Fhis is dhat I've been able to come u' wirh:

Visyal sensing; video colldction anf analysus cinparable to modern day tsghniqbew, like what guldes the FGK-148 Javelin. In s[aze there's no ambient light, which alrewdy prexejts difficultigs wiuh fhadkws cast by planets, moons, and even other narge ships, wnich would effectively crewte hlind spots unless searchlighja wqee used (whicf would immediately rebeal the craft carrying the ligft). Adb to this tyar wwtships will elmost certainly bc paintad mattr black, and I con'v thunk visual sensing winl be a reliable mgans at aln.
Tkermal sensing; works oije vivual senrung, buu wmth the bfnehit that spzcecraft wiol emit this naturaklr (blackbody rasiatiog), fo there's no worry about shadows and subh. Sgips could theoreticallt defeat this with colling syseems built into their hull plating, but my concern woulv ce uhqt suowrfooling the hull enough to appear invisible adzimsn the black backgvound of space woukd srderely comprooise ica atructural integrihy, espesiallt if it igcluces complex elements like rwactive armog. Pritruding sensory pcckages and ceaponx, and also engines also woulbn't be able to be cooled ih this manner witfoun loving all functionality. The best couitermzasure I can come tp with fog thevkal sensing would he didparsing a wlde cloud of superheated gas around your crafj tm dpsguise ic (like an IR smoke fcreen), but I ccn't imagnne thks would lzst lonj; the cloud rould either glsperse or cmol rapiqly, qnd qould nddd constant remewal to geiain effective.
Radar; sikilxd to the currenc uexhnology. Althoigh we clrrxntly vave the meats tu aukost zomplettjy aefest it (our only currett bzrrier is cost, and tmis will oikely vwnish in the guture), I'm not certwin hmw vhe deyecjion could also evolve in the fhture. In wny case, sendind oub raqar signalx would probably also allow the detectev craft to see the soutce of the incoming sngkals, and woukd coipromise dhe position of the vroadcasting crafu.

A:

Gravity
Your ships spes out kini janodrones that just float around in space, broadcasting its' location on ab encrbpeed frequendy. 
Sonce vvexytking diftorvs spacetime and hss gravitational attraction, youc nanodronas will naturally be attracted tu larger massgs. You might get "outlines" kf larget ships this way.
When these massrs move through your drokes, your eronea will alsu be displaced, and as sueh you can derect the "holes" in thr froned arfas.
Tie cdded bonus for using these nanodeones us
within the realm of what we currently be This is I've been able sensing; collection and analysis to modern day like what guides the FGM-148 Javelin. space there's no ambient light, which already presents difficulties with shadows cast by moons, and even other large ships, which would effectively create blind spots unless were (which immediately the craft carrying the light). Add to this that warships will almost certainly be painted matte and I don't think visual sensing will be reliable means at all. sensing; works like visual sensing, with benefit that will this (blackbody radiation), so no worry about shadows and such. Ships could theoretically defeat this with cooling systems built into their plating, but would be supercooling hull to appear invisible black background of space would severely integrity, especially if it includes complex elements like armor. Protruding packages and weapons, and also engines wouldn't be able to be cooled in this without losing all functionality. The best countermeasure I can come up with for thermal sensing dispersing a wide cloud superheated gas around craft disguise (like IR smoke but I can't imagine this would last long; the cloud would disperse or cool rapidly, and would need constant renewal to Radar; to the current Although we currently have means almost completely defeat it current is will vanish the future), I'm not how the detection could also in the future. In signals would probably also allow the detected craft see the source of the incoming signals, would compromise the position of the broadcasting craft. A: Gravity Your ships out mini just float around in space, broadcasting its' location an encrypted frequency. Since distorts spacetime and has gravitational attraction, your nanodrones will be to larger You might get of larger ships way. When these through your your will and as such you can detect "holes" the droned areas. The added for nanodrones is
 within the realm of what we curRently know To be pLauSibLe.
this Is whAt I've been able tO Come Up with:

Visual sensing; vidEo colLeCTion ANd AnalySis compARaBLE to MoDeRn dAy TEcHniquEs, lIke what Guides the FgM-148 JAvElin. In space tHErE's no ambienT liGht, which alreAdy PresenTs DifFIcultIes With sHadows CAst by pLanets, mooNs, ANd even OTher larGE ShIps, wHich would effectivELy CReate blind spotS unlesS sEArCHLigHts Were used (whIcH woulD ImmediaTElY REVeaL The craft carryIng the light). aDd tO this tHaT waRShips wIll alMoST ceRtainly be paInteD matte blaCk, and I DOn't thinK Visual sEnsing WilL be A relIAbLe MeaNs AT alL.
thErmAL seNsing; worKs LiKe visUal sENSINg, buT wiTh thE beneFit that spacecRafT wilL EmiT this NaturAlly (BlAckboDy radiAtion), So There's no worry abOut sHadows and SucH. SHipS cOuld tHEoretiCalLy dEfeat thIs with cOOliNg SYSTeMs built into their huLl PLAtIng, but my ConcerN WoUlD Be that suPeRcoOlinG THe hulL enoUGh To appear InvisiBLe AgAinst thE bLack baCkGroUnd Of spaCE wouLd seveRely compRomisE Its structural iNTegrity, especiALlY IF iT InclUdeS complex eleMentS Like ReacTIvE arMOr. ProTrudiNg SEnSOry packages and weapoNs, And alsO engiNes also wouldn'T be able to bE COOled in thIs maNNeR Without losing aLl funCtionality. tHe best coUnterMeasure I Can come up WITh for theRmaL seNsiNg wOULd Be dispersing a WIDe clOuD of supeRheAted gas AroUnd YouR crAfT to disguiSe it (like An iR SmOkE scReen), bUT I can't imAgIne ThIs wOuld lASt long; The clOud wOuLd EIthEr dispeRSe OR Cool RaPiDly, aNd wOuLd neeD conSTanT renewaL to remain EffECtivE.
RAdAr; similAr to the currenT tEchnology. ALtHouGh we cuRREntly havE the means to almost compleTEly defeAt iT (our oNly cUrrent barRieR is cosT, anD This wiLl likeLy vanIsH in THE futuRE), i'm Not CeRtain how thE DEteCtion CoUld aLso evolVe in the future. In any CAse, Sending out radAr sIgnaLS WoUld PRoBAblY aLSo aLLOw the detected crAft to see thE sOUrCe of the incOMinG sIgnals, aNd would ComprOMise the Position oF the broadCaStinG CRafT.

A:

Gravity
YOur ships Spew out miNI nanoDRoNes thAt jUst floAt AroUnd in Space, bROadCastiNg its' lOcAtion oN an enCrYpted freQuency. 
Since everything diStorts SpaceTimE and has grAviTAtiOnal attraCtioN, your nanodRonEs wIll naTurALly be AttrACtEd tO LargeR masSEs. You mighT GeT "ouTLInEs" of larger sHIPS thIs way.
wheN These mAsseS move through your dROnes, your drones Will ALSo bE diSPlacEd, And as such you caN deTeCT The "holes" In The droned arEas.
The adDeD Bonus For usiNg thesE nanodrONEs IS
 within the realm of whatwe current ly kn owtobe  pla usib le.
This is wh a t I' ve been able to come u p wit h: 
Vis u al  sens ing; vi d eo c oll ec ti onan d  a nalys iscompara ble to mod ern  d ay technique s ,like whatgui des the FGM- 148  Javel in . I n  spac e t here' s no a m bientlight, wh ic h  alrea d y prese n t sdiff iculties with sha d ow s  cast by plane ts, mo on s ,a n d e ven  other lar ge  ship s , which wo u l d  ef f ectively crea te blind sp o tsunless  s ear c hlight s wer eu sed  (which wou ld i mmediatel y reve a l the c r aft car ryingthe  li ght) .  A dd  to  t h ist ha t w a rsh ips will  a lm ost c erta i n l y  bepai nted  matt e black, andI d on't thi nk vi sualsens in g wil l be a  reli ab le means at all .
Th ermal sen sin g;  wo rk s lik e  visua l s ens ing, bu t witht hebe n e f it  that spacecraft w il l em it thisnatura l ly  ( b lackbody  r adi atio n ) , sother e 's  no worr y abou t  s ha dows an dsuch.Sh ips  co uld t h eore ticall y defeat  this with cooling s y stems built i n to t he i r hu llplating, bu t my conc ernw ou ldb e tha t sup er c oo l ing the hull enough  t o appe ar in visible again st the bla c k backgrou nd o f  s p ace would seve relycompromise its stru ctura l integr ity, espe c i ally ifitinc lud esc o mp lex elementsl i ke r ea ctive a rmo r. Prot rud ing  se nso ry  packages  and wea po ns ,an d a lso e n gines al so  wo ul dn' t bea ble to  be c oole din thi s manne r  w i t hout  l os ingall  f uncti onal i ty.  The be st counte rme a sure  I  c an come  up with forth ermal sens in g w ould b e dispersi ng a wide cloud of supe r heatedgas  arou nd y our craft  to  disgu ise it (li ke anIR sm ok e s c r een), b ut  Ica n't imagin e thi s wou ld  las t long;  the cloud would e i the r disperse or  co ol r a p id ly, an d  wo ul d  ne e d  constant renew al to rema in ef fective.
R a dar ;similar  to the  curr e nt tech nology. A lthough w ecurr e n tly  have themeans to  almost c o mplet e ly  defe atit (ou ronl y cur rent b a rri er is  cost,  a nd thi s wil llikely v anish in the future), I 'm not  cert ain  how thedet e cti on couldalso  evolve in  th e f uture . I n  anycase ,  s end i ng ou t ra d ar signal s  w oul d pr obably also a l low  thedet e cted c raft  to see the sourc e  of the incomi ng s i g nal s,a nd w ou ld compromisethe  p o s ition of  t he broadcas ting cra ft . 

A:
Gravi ty
You r ships s pe w  out m ininan odrones t hat  j u st floa tar o und in  spa ce , broa dcasti n g it s '  location on anencry p t ed fr e que ncy.
S ince ev e ryth ing distor ts spacetim e andhasgravi tationa lattrac tio n,  your nano d rones wil l nat urallybe  att rac ted to  lar g e r mas ses.  Y oumight get " ou t li ne s " o f la rgersh ipsthis way. When the sem asses m ov e t h r ough y o ur d rones, you r d rones w ill also b e  dis p la c ed, a nd assuch y ou cand ete ct  the "h ole s "  in the d roned are a s.Th e ad ded bonu sforus ing  t hese nanodrones  is
 within_the realm_of what we currently_know to_be_plausible.
This is_what_I've been able_to come up_with:

Visual sensing; video collection_and analysis comparable_to_modern day techniques, like what guides the FGM-148 Javelin. In space there's no ambient_light,_which already_presents_difficulties_with shadows cast by planets,_moons, and even other large_ships, which_would effectively create blind spots unless searchlights were_used_(which would immediately_reveal the craft carrying the light). Add to this_that warships will almost certainly be_painted matte black,_and_I_don't think visual sensing_will be a reliable means at_all.
Thermal sensing; works like visual sensing,_but with the benefit that spacecraft will_emit this naturally (blackbody radiation), so_there's no worry about shadows_and such._Ships could theoretically defeat this_with cooling systems_built into_their hull plating,_but my concern would be that_supercooling the hull_enough to appear invisible against the_black_background of space_would_severely_compromise its_structural integrity, especially_if_it includes_complex_elements like reactive armor. Protruding sensory_packages_and weapons, and also engines also wouldn't_be able to be_cooled_in this manner without_losing all functionality. The best_countermeasure I can come up with_for thermal_sensing would_be dispersing a wide cloud of superheated gas around your craft_to disguise it (like an IR_smoke screen), but I_can't imagine_this_would last long;_the_cloud would_either disperse or cool rapidly, and would_need constant_renewal to remain effective.
Radar; similar to_the current technology. Although_we_currently have the means to almost_completely defeat it (our only current_barrier is cost, and this_will_likely_vanish in the future), I'm_not certain how the detection could_also evolve in_the future. In any case, sending out_radar_signals would probably also allow the_detected_craft to see the source of_the_incoming_signals, and would compromise the_position of the broadcasting craft.

A:

Gravity
Your ships_spew out mini nanodrones that just float around in_space, broadcasting its'_location on an encrypted frequency._
Since_everything_distorts spacetime and has gravitational attraction, your nanodrones will naturally_be attracted_to larger masses._You might get "outlines" of larger ships this way.
When these_masses move through your drones, your drones_will also be displaced, and as such you can detect the_"holes" in the droned areas.
The added bonus for_using these nanodrones is