I have only seen one, ever. I can't even remember where, I just remember looking out the window on the highway in the middle of the night, and I saw red and blue flashing lights down the road. Once I finally got close to it, I noticed it was a plain black VW beetle unmarked car....
I guess they sometimes get government seized cars and throw some lights on there to try to surprise the people that are breaking traffic laws... but don't quote me on taking gov seized cars... Though it wouldn't be a bad idea, taking an almost free car and slapping on some lights, and a siren... There you have a cheap/possibly almost free police car. Instead of buying brand new Crown Vics and Chargers and modifying them to hell...