Do you know that the thank-you note made it to them int he mail?
I might bring it up directly to them, in front of lots of people, w/ plenty of volume.
"Oh, it's so nice to see you. We haven't seen you since so long, when we were at X event, and we thanked you for the gift cards we used to buy our camera with. You know, we still think of you when we use that camera. You *DID* get our thank-you note in the mail, right? I'd be so upset if you hadn't received it, and didn't know how much we appreciated your present."
And I'd feel free to say to anyone who ever brought you that news, in a very bewildered tone, "We thanked them in person AND we sent them a thank-you card. I don't know what else to do!"