Police can legally hide tracking devices on your vehicle, thus sayeth the court system.
The Seventh Circuit of the US Court of Appeals has decided that it does not violate any of your rights for police to place a GPS tracking unit on your vehicle if they have probable cause. The case in question has to do with someone whose car was tagged with a locator beacon by the police because they thought that he was up to something. He says that it violated his fourth amendment right to freedom from unwarranted search and seizure. The thing is, it wasn't a search or a seizure. In fact, I'd say it was no different from parking enforcement chalking your tires to see if you've moved your car since their last trip around the block. Besides that, 'reasonable suspicion' is well-defined insofar as law and law enforcement are concerned, and has been for a couple of decades now.
A bit about how these things work, because there are a lot of misconceptions about them (and because I feel the need to geek out a little). They consist of a tiny GPS receiver that can determine its own location, and thus the location of whatever they're attached to. Then they broadcast the current coordinates of the GPS receiver on a little-used radio frequency; the transmission might be encrypted, it might not. I don't know because I haven't actually gotten my hands on one of these little buggers.