I have a similar story, but I was playing on the beach. There was a mound right next to it and I would love to play there, and the mound had some funny stones. One of them was square with something painted on it, I was fascinated by romans so I annoyed my parents with "I found a mosaic!" and took it with me.

Turns out, years later, they excavated a roman villa there. Funnily enough, the same beach has roman villas, dinosaur prints, austro-hungarian tunnels and yugoslavian bunkers. Quite a lot of history in one pretty beach.