Radar detectors DO emit a signal that can be detected (hence the FCC license number on them). The most popular "radar detector detector" is called VG-2. Many models of detectors offer "VG-2 detection" where they shut off when they detect the detector detector (confused yet?). I had one once with that "feature", but it falsed all them time and I always had it shut off. All the tests I've seen have shown that the VG-2 doesn't pick up the V1. Don't know much about the newer technology of detector detector.
