Do you really think that the police mark their unmarked vehicles with stickers that say “Unmarked police car”?
Nope. But do I think some nut might be putting stickers on his car and playing cop. Probably not. But it's a possibility and I don't want to be the patrolman who ignored it if it turns out he was. It's close but I can see a cop thinking he had to check it out. Make it as quick as possible, be polite, case closed. If they were being real dicks about the whole thing, then that's a different story altogether.