I found this on another site. See the 4th post in this thread.
Comments?1) ... We have a few specially tuned Kustom Signals Hawks that were provided to the dept with accompanying VG-2s, the radars are tuned off frequency and those will not set off even some top of the line radar detectors. We we given them to conduct a study dealing with radar detector use, we never had to give them back, they are licensed, we still have them in service.
There's also a number of claims in that thread that a radar gun can register a speed (presumably in non-POP mode) and not trigger an alert from a "top of the line" radar gun. Cobra maybe?