This discussion of finding the device makes me wonder how big they are and what they look like and where they get their power? A GPS has to have an antenna with a view of at least a slice of open sky, and I don't know of any really tiny ones. A GPS receiver itself doesn't need much power, and could run for some time on a small battery if it doesn't have a display to support. A transmitter which could send the info back to the police is another story. Continuous transmission over any distance, even using fairly low power, is going to eat up any small battery fairly quickly. Intermittent transmission would make it last longer, but unless you have a pretty good sized battery, it still wouldn't last all that long before needing replacement.
Transmitters and receivers can be made pretty small, but not invisible, and a transmitter needs an antenna as well.
Basically, any useful device would be pretty hard to conceal and secure in place, and it might need to tap into the vehicle's 12 volt system for power. I have two old trucks, and enough problems with the wiring and corrosion, thank you very much. If someone is going to decide to screw with the frame or paneling of my vehicles to attach something, or screw around with the wiring to drain power, I think that someone should be a judge with a reason that will stand up in court, not a cop with a hunch.