You are right. You can’t rely on the effectiveness of these jamming devices.
If you’re really serious about not getting tracked, then you should get one of those undercarriage inspection mirrors, a kind of mirror on wheels with a handle that border guards and building guards use to look for bombs strapped to the undersides of vehicles.
I can’t imagine anybody tracking me and my commute is by subway and anybody who wants to know where I go every day could probably find out in about 5 minutes but if I used my car more often or lived in a western state and had any reason to think these creepy crawly manson-like fed-agents were going to stick something on the bottom of my car, I’d get a mirror and inspect my car every once in awhile.
But then I guess the question becomes, what do you do if you find one of those devices? If you touch it, are you breaking the law?
I would summize that any “object” attached to you or a piece of property in your posession could be removed without penalty...
But if you do so, know that whomever “is” watching and tracking you will know you did so, and you may just get that “knock on yer door”...
And if they are really watching you, there will be more than just a GPS tracker device somewhere around you...
Not that I am inciting paranoia or anything...;-)