@Nw This is a serious invasion of your privacy if it has been done to you and should be reported to the police asap.
The most likely spots they may be hidden would be the main or aux fuse box, most likely the cars OBD2 port (usually below or near the steering column), or it may be hidden more sneakily like in the boot compartment, glove compartment, beneath a seat perhaps.
Best way to detect would be to go to an autoelectrician. If that is not possible, you can get a cheap RFID detector from amazon. Turn your car's electronics off and scan. If it beeps, you may have gps installed without your consent. If you are worried about electronic bugs in a room, easiest way to sweep for them is using your phone's camera - it can see infra-red, your eyes cannot. Hold your TV remote button down (any button is fine) and take a picture - you should see the red light on the camera photo. Hidden cameras emit the same. RFID scanners will also help in this regard, same as finding hidden GPS. Hope this helps.
