Via Cryptogon and Wired:
This guy in San Jose found TWO GPS trackers on his car (one after he found and took pictures of the other one) and a strange man was seen poking around his girlfriend's car, which makes them think they were retrieving one off her car before she could find it. Also the police showed up while he was talking to Wired.
I thought I'd put some pictures from it so you can know what to look for. He found the second one when he saw "what looked like a cell phone antenna inside a hole on his back bumper".
Incidentally you can also get someone's general location from their cell phone, and many cell phones also have GPS, and some of them will broadcast even when turned off, and even when the battery is out, because they have other batteries as well. So keep that in mind too.
If that's not enough, there are RFID's in tires that can be tracked by boxes on some roadside lamp poles. I'm assuming that's only in or near large cities or interstates, at least for now. And if you decide to walk and leave all electronics behind, now they're coming out with secret-camera light poles.
Maybe we should all take to wearing a large brimmed hat and a domino? Then they'd have a hard time telling any of us apart on the street. Or burkas for everyone even the men.