Ya know, I've been on both ends of the "Cop coming up to a parked car" scenario, having been a Police Officer.
The most memorable was the time my partner and I were riding past a club and noticed a car parked all the way in the back of the parking lot who's tail lights were flashing irregularly. We shut off our lights and rolled up to the vehicle. It was a hatchback with the big window in the back and when we got behind it and turned on our headlights, spotlight, and flashing lights, we got a fantastic view of a guy on top of his girl in the drivers seat. We caught him in mid-stroke and the look on his face was absolutely priceless. It seems that he had his foot on the brake pedal and didn't know it so with every thrust, he activated his brake lights.
I still giggle to this day when I think of the look on his face when we lit up.