Following the ruling in January, the FBI has deactivated some 3,000 GPS units that were potentially infringing on the Fourth Amendment. The decision seems to be making waves in the U.S. Justice Department. Andrew Weissmann, FBI General Counsel, says some of the devices have been difficult to retrieve, as the vehicles they were once tracking now move undetected. The FBI has sought temporary permission to reactivate some of the devices to locate and retrieve the hardware. Weissmann says the FBI is also developing new guidelines regarding the legality of its agent's actions -- from the application and use of tracking devices, to the extent a suspect's garbage can be searched before the agent is committing trespass.
In short, they are trying really hard to not infringe on your privacy rights.
And if you do find a weird device in your car, and were involved with the law before, you might want to give the FBI/local law enforcement. They would probably like it back.