Suppose someone was to develop a FHSS RADAR gun. Frequency Hopping Spread Spectrum radios only transmit on a particular frequency for a few milliseconds, then switch to a new frequency in a pseudo-random hopping pattern. It would be like Quick Trigger on steroids!
Even if you know that FHSS was in use, and even if you also know the hopping pattern, FHSS RADAR would be almost impossible to detect. A cop could leave this sucker blasting constantly and never trigger any current RADAR detector.
The good news: this is an old idea, and had been around for almost 2 decades. The fact that such a thing does not already exist means it's either too difficult or too expensive to build and/or get licensed. Or perhaps no one bothers because LIDAR does a much better job.