My sisters rescue daschund is OCD with rocks. They got rid of every rock they could find in their yard, now he digs to find them. My sister brought him out on thanksgiving and the first thing he did was go straight for my parent landscape stones. I don't know at what point a fun game with a rock turns into an OCD behavior, her dog was like this when she got him. He plays just as yours does, but like bailiff said, you seem to have a pretty good handle on things, so probably not an issue.