Radar detectors give off similar-band radiation, which is why they can be detected. The better ones are higher gain and more effectively shielded, so their emission signature is lower, but they can often be detected too. Just like anything else, it's all down to the technology you're up against. Better funded police departments might have much more up to date equipment than Hillbilly County PD.. The best defense is to not use one, or not whine when you're caught.