The case of Yasir Afifi should alarm us all: the FBI secretly attached a GPS device to this young man's car so they could track his movements. Afifi is a 20 year old computer salesman and college student. (There are many news articles about this story, just google it.) A Federal court had already ruled that the use of GPS tracking devices (without the suspect's consent) does not require a court order as long as the tracking (for now, it seems these cases involve automobiles) is happening on public property. It seems to me, however, that the physical act of placing the tracking device on one's private property is, in itself, a gross violation of privacy. How is it that the police can go messing around with one's car without a court's permission?
In this case Afifi found out he was being surveillanced when the guy changing his oil noticed an odd wire, which led to the gps device. They posted images online asking for help in identifying the strange object. Then the FBI shows up at Afifi's door two days later asking for their spying equipment back. And you're going to give it back? No way! If something is attached to your car, it's now yours!