I remember an incident in Indiana. I was driving on SR 37 between Bloomington and Indianapolis. I was doing the limit (55) and an unmarked car pulled out and followed me for several miles. I set the cruise at 55, the unmarked car kept following waiting for an opportunity to pounce. He finally got bored and turned around. That is the kind of behavior I am talking about. Yes, a marked car could do this too, but I've never observed them doing it.