I've always had the habit of scanning traffic while driving to spot unmarked police vehicles. Not particularly because I'm speeding but for the same reasons I am aware of surveillance cameras nearby.
Maybe it's a game I play to keep my mind from wondering. On the way in to work this morning I counted 9 tells that I typically use to spot an unmarked car that is behind me.
One of the tells even signals if it's a state or federal owned, which is handy if you're wanted by the FBI.