A miniature camera housed in the front and rear of the car. When ever contact is made to the bumpers a picture is taken. 5 times my A4 has been bumped (just slightly) in car parks and no witnesses! No body damage, but in each case paint has been scraped off down to the bare plastic.
Now I tend to wait until I have a few and then get it all done at once to save costs. I currently have a mark on the front bumper from Tescos car park. I think I am a marked man.