This really annoys me!!!!!!
It happened to my GTE years ago, left it round the back at work, came out at 2am and noticed a MASSIVE scrape all the way down one side!!!!!!

Car was worth around £4000 at the time too and i only had third party insurance!
Anyway, next day i went round all the cars (around 50 with a fine tooth comb as it was bound to have damaged their bumper/bodwork, couldnt find anything but noticed a big 7 series beemer inside a loading bay which seemed very strange. Went in for a look and what do ya know, the remains of my paint all over the bumper and wing!!!!!
Turned out it was the CO-OP managers car! Stormed straight into the offices and dragged him outside. He tried to deny it, but when i pointed out my GTE was a special paint job so a very rare paint, he admitted it!

Ended up getting it repaired for me and i got a full re-spray out of it, got it de-locked with a few other bits and bobs doing at a cost of nearly £3000, so didnt turn out too bad in the end really.
But still, thats not the point!
Well worth looking round the car park everyday for a few days at least mate.