Well known. They watch for front end's to dip, which tells them either a) driver has radar detector, or b) driver saw cop and knew he/she was speeding.

Either way gives them probably cause to stop you. You may not know this, but at least here in VA, radar alone is NOT probably cause to stop you for speeding. The cops have to go to long training classes on estimating vehicle speeds, and when you go to court the cop will tell the judge he estimated your speed at X, and verified your speed with radar. The cops get good enough at this training to be able to estimate within 5 MPH, and have to pass a test doing so or they don't get radar certified.