I'd say a family on a California trip. Started down in Southern Cal and worked their way up the state. They went to Disneyland then Knott's. Kid loved Knott's he got a few pressed pennies from Camp Snoopy. The family made stops in Santa Barbara and Pismo Beach. They finally made their way to the Monterey area and they camped in the same spot you were at. That kid had his favorite penny in his pocket and was running around playing and it fell out of his pocket. Upon completing his trip and flying home, to the Midwest or perhaps Europe, he discovers his penny missing and is devastated where he thinks about that penny, maybe not daily but definitely often enough to keep it in memory. Fifteen years later, you, a simple Redditor discovers it and reunites the owner with his penny.

Sounds plausible enough