I think a great way to deal with this problem in a hacker type of way would be to create and post schematics for a cheap device (e.g. using your old cell phone or other types of old consumer electronics devices) to detect the presence of these devices. A big motivation of using these covert devices is that they work, i.e. the average tracked person is unaware of them. If this fact can be nullified, then law enforcement people may be less willing to use such tactics.
GPS signals are everywhere, so the only thing you could intercept would be the outgoing transponder signal. This could be masked as almost anything.... random cell phone packets, radio waves with local pickup, etc. Hell, they could even just manually retrieve the data once in a while when your car is parked outside your house.
So it might be hard to execute, but certainly still a good idea. I just wanted to point out that GPS logging is entirely passive; only the transmission can be detected.
