They send six agents to your home to demand it back, obviously.

From Gizmodo:

A California student got a visit from the FBI this week after he found a secret GPS tracking device on his car, and a friend posted photos of it online. The post prompted wide speculation about whether the device was real, whether the young Arab-American was being targeted in a terrorism investigation and what the authorities would do.

It took just 24 hours to find out: The device was real, the student was being secretly tracked and the FBI wanted their expensive device back, the student told Wired.com in an interview Wednesday.

The answer came when half-a-dozen FBI agents and police officers appeared at Yasir Afifi’s apartment complex in Santa Clara, California, on Tuesday demanding he return the device.