Did you know that radar will reflect off of hard surfaces other than cars, like street signs? Radar hits a street sign, you get a weak alert on your RD ahead but as you get further down the road you don't see anything. A little bit further you see flashing lights in the distance and the signal has stopped...he's already got one. Or maybe you don't see anything but the signal strength stays about the same. He's moving and the reflections are still bouncing and echoeing back to you.
BTW - I posted this so that if anyone else has some "DID YOU KNOW" info on radar and detectors we could have a thread.