I feel for you dude.
After parking my car(s), I now take a quick (but in-focus) cellphone picture of each car's licence plate that is parked beside me and a photo of my door panels using the reflections to show the unmarked finish. And when I get back to the car (no issues so far - fingers crossed!), I delete the photos of the other cars. Simple.
However, waaaay too OCD my wife says...
I still park in spots well away from minivans, cars with booster seats, beaters, and will always prefer to park next to other higher-end cars (P-cars, well-kept Mercs, etc)...that includes paying more to park closer to the airport terminal so my beast is safely with other higher-end cars.
Funny though - it seems like there is a whole risk-assessment science in owning and parking nice cars in public. Who knew?
SF