A Radar Detector Detector uses the same characteristics as a radar detector. Both search for a certain frequency to warn the operator that a device is being used. In this case, a radar detector detector searches for a radar detector in operation. Since all electronic devices emits some type of an electronic signature, this is what a detector detector is tuned for, a detectors Local Oscillator.
There are many forms of Radar Detector Detectors. There are either a specially made unit or a modified radar detector to search for radar detectors. Two that are in operations are the VG-2 Interceptor and the Stalcar. Others are actual radar detectors that are tuned to look for radar detectors. If you have run a detector before and had it good off as you pass another vehicle, this is called cross talk. This is what a tuned detector searches for.