License plate reader cameras could soon be joined by sensors that detect phones inside passing cars. As it passes by a license plate reader, the camera records the car, which can be linked through vehicle records to its registered owner. SignalTrace, a system marketed by the security company Leonardo, is designed to work alongside automatic license plate readers. The company says it can recognize groups of consumer devices that regularly move together, then associate them with license plate records and time-stamped locations. License plate readers help police track vehicles -- and raise significant privacy concerns.