Suggestions to filter RF from a long wire connected to a logic gate
3.3 MOhms - can't be serious! A drop of water bypasses your wire and nothing is detected. Change the pullup resistor to 500 Ohm. If that's too much in current consumption, consider to have pulsed operation -  say 1 ms in every half second.

No test? Again: Cant't be serious! You should have a relay that makes a test breakage somewhere, preferably in the middle of the loop and a checking plan, too, preferably automatic.

A lowpass RC filter at the input of your nand gate kills the RF, have a time constant 1...100ms.

Consider  more specific detection.  ON-OFF does not see the bypassing. At least make total bypassing difficult by keeping the outside ends of the loop unreachable at the same time.

ADDENDUM: 

The test relay: A normally closed switch is inserted as a part of your loop. The switch is remotely controlled.The simplest form of it is a relay. If opening the switch does not cause alarm, the current has a bypass way somewhere (=a bypass circuit is inserted, broken insulations and the vire is in the water)

Before any further circuit analysis a threat analysis should be performed. The most important things after deciding that the intrusion trial is probable and a tripwire is useful:


does the intruder know beforehand that there's a tripwire (=by having some inside knowledge)
can the intruder survey freely the systems as long as he wants (=no dogs, no cameras, no motion sensors)
if no foreknowledge, can the intruder see the tripwire before it's too late
what's the workaround skill level of the intruder 


I spotted word "BIFILAR" in your text. Your loop seems to have 2 parallel wires. Thus it can have 4 wire ends, all available where your electronics reside. See the illustration:



The bifilarity (=2 parallel identical wires) gives a lot of new possiblities that you should explore


In heavy disturbing fields (=powerline, radio transmitters, high current works around) your system can feed the test voltage between the 2 wires. Any disturbing signal that affects equally to both wires can be cancelled
If there appears a resistance difference  between the wires, something is added to the circuit
if there appears a connection between the wires, something is added to the circuit
a voltage or intentional signal between the wires can be used to do something useful. The wires can be at the same time for power supply, signal transmission and your original tripwire loop
by measuring pulse reflections the place of the breakage can be deduced
(not useful if the max distances are only a few meters, but very useful if the distance is beyond the field of the vision)


I have not written about how to protect against the lightning (not direct hit, but nearby). For that the methods can be borrowed from the landline telephony.

A way to reduce induced voltages - not much better than a twisted pair as a single wire loop and makes bypassing easy. Gathers common mode (=against the earth) voltages as easily as a single wire loop => NOT RECOMMENDED. To utilize properly the 4 end loop, you must have more complex circuit.