They can detect some kinds of devices. Don't know which ones, but when I lived in VA, I had a friend who didn't even know her husband had installed a detector in her Honda Accord and she got stopped because the LEO detected a radar detector on her car. He had her open the hood and finally found it after a little bit of a search. He didn't cite her, because it was obvious to the LEO she had no clue it was there, but he did take the detector.