Breaker tripped under no load?
This more of a comment than an answer, but I can't post comments because I don't have enough brownie points yet!

Anyway, had an identical problem at a home I used to live at.  First symptom was the circuit supplying the freezer in the garage kept tripping, so I thought dang, bad freezer. But plugged into another circuit and it was fine, no tripping.  The circuit the freezer used to be plugged into kept tripping.   Then we lost some outlets and lights in the living room (shared wall with attached garage).   

OK, now what!?  Crawled up in the attic and found the cabling where it went thru the top plate had been chewed thru my mice!  There was a dead one right there in the hole.  But it only explained part of the problem because the living room lights were on a separate circuit.  I pulled some outlets to do continuity checks and they failed.   So I pulled the sheetrock off the garage side of the shared wall and found more mice had chewed thru cables along with a large mouse nest.  The electrocuted bodies where still there.  I counted my lucky stars the breakers did their job and my house didn't burn down.  Had to do a reasonable amount of re-wiring,  but again, very glad I had a Square D QO panel and not Zinsco panel! 

I share this story because it sounds similar to your problem. Shorts and tripped breakers without known reasons are scary and cause for concern.  Good Luck.