When a cell phone is being used, it emits radio frequencies that can be picked up and detected. The frequency varies depending on what the phone is being used for—data, voice calling and, of course, texting—which can give a person away if they happen to be secretly tapping away at their phone out of view of any passersby.
A company in Virginia called ComSonics wants to turn this type of sensor into a pointable device that could be used by law enforcement in the same way a radar gun is. If an officer could target your vehicle with the device and detect a texting signal, they might be able to pull you over without ever actually seeing the violation take place.
Privacy hawks take note: The device would only be able to sense that a phone is being used for a specific purpose, and it cannot record or translate that signal into something readable. It’s simply designed to detect.
https://www.dailydot.com/debug/texting-while-driving-gun/