Samething happened to me, not at a car meet, but in ASDA car park.. while I was shopping.

Went back in and told them someone had stolen part of my car, security checked CCTV and gave a copy to the Police.

The police managed to get the cover back from the person that took it! I was so surprised, usually the Police are useless when it comes to stuff like that.

Make sure you tell them its an expensive/rare part from an old car .. so they take you seriously.