"The £999 system (plus £100-£200 for installation) monitors the frequencies of emergency services’ Tetra and Airwave radios and, according to the manufacturer, listens for the distinctive regular pulses sent by the radios every four seconds. These could be emitted by marked or unmarked vehicles; by radios worn by beat officers or mobile speed camera operators; or by radios in police helicopters. When a transmission is detected, the driver is alerted."And I agree they really wouldn't like this if someone created a network of dectors and publicized their locations.
Via Slashdot