every move you make. in yasir's case it's not clear where his car was when he said fbi agents placed a tracking device on it. a computer salesman in santa clara, california, removed the device when he found it. it wasn't long before he said fbi agents approached him asking pointed questions. >> have you ever been to yemen for any type of training? >> no. >> reporter: the agents asked for their gps back. afifi has not been accused of wrongdoing. the fbi wouldn't comment on the case. they don't need a warrant to track a vehicle with a gps when it's in a public space. how watchful an eye could this device keep on you. >> what's the bess place? >> the frame of the car. the truck we're here with. >> reporter: i recently tested a gps tracker on our cnn vehicle with veteran private investigators mike o'connell. he's been using them for years,