Hey, Duck in the Fog! Sounds like it might be his if you weren't advertising that you found it, and you were in the same general area as where you found the lost treasure. But, legally or not, if I was convinced that it once belonged to this other person I would have to give it back. That's just the way I am.

I guess if I were in the same situation, I would hope someone would return it to me and give me the option to take it back or offer it up to the finder. Along with a special thanks for being the type of person the world needs more of..... Good Luck on your decision.