The frequency it tunes to. The radar detectors have an internal oscillator tuned to a frequency that downconverts the police radar band signals to an intermediate frequency, which in turn is processed to make a decision...did it detect a radar signal, or just background noise? There may be two stages of downconversion, meaning two oscillators. These oscillator operate at fixed (and easily calculated or measured) frequencies. These oscillating signals will radiate out from the radar detector, albeit at very low power levels. These frequencies will probably not be common with any other devices. A highly sensitive receiver tuned to these frequencies can pick them out. Once they get a "lock" on your detector, you're busted.
