The hard part, as has been discussed on OG before is ID'g your signs. Without pictures of which signs and whether there's specific damage unless someone is offered both signs at once it's difficult to know when they turn up. The Used Cars is actually easier to ID being less common, so hopefully it will get spotted.
With an ID of area of theft being midwest I know the chance they will turn up in SC is slim. However, if I was in the specific region you live in I'd know to be seriously on the lookout. I'll still keep my eyes out here in SC though.
The reason I recovered 80% of what was stolen from me last November was because I gave specific information where, when, what was stolen and my local contacts had their ears and eyes out knowing exactly what to look for. I spammed Craigslist, OG, and regional Facebook sites so that people were on the lookout all over in case it was a professional hit instead of just opportunistic.
Proactive securing of signs for anyone with displays outside should be the norm from the get-go, including video/camera surveillance. If you can afford to display a Flying A, you should be just as willing to invest in reasonable security. (multiple securing methods to start).
Photo-logging of entire collections should be commonplace, you can buy CHEAP digital cameras so there is no excuse for not having evidence of your collection for reference and insurance if you've got money invested.
I hope the OP keeps us posted on the progress as well. It's good to know what to be on the lookout for at shows.