Both LawMate devices the RD-10 and RD-30 can pickup 3G/4G signals. Most GPS trackers go to sleep after 10 mins of inactivity. 


When conducting a sweep of your car be sure to turn your mobile phone off and drive to a less residential area as WiFi signals can also be picked up by the Detector.


It's also worth using a Hoist to check the under side of the vehicle if at all possible.