Since this thread started, there have been GPS trackers introduced that work for musical instruments. The main challenge was developing a battery that could last longer than a few days.

The micro-GPS is right now the best solution to track music instruments - AND tour trailers. It's small enough that you can hide it inside a case and lasts for about 2 weeks (recent test I did lasted three weeks.) This all depends on how much the GPS unit is moved (thus triggering the motion alert) and if the signal needs to work harder (if the GPS is in an all metal trailer).

Check my site www.ScreamingStone.com for a review.

Rock on, and protect your instruments.