Was it just a tip? or an actual Muffler. Cuz if it was the actual muffler it might be a personal car, and I dont think he could pull you over, cuz his car is illegal. But I have heard of this...cops in unmarked cars that usually have rims and dark window tint. They use cars that were taken away and try to attract street racers. But from my understanding they cannot initiate the "race" and they can't have anything illegal themselves. I maybe wrong on this.