So did I. But I just looked it up to see if there was an official procedure. IIRC the military ID says to drop it in the mail. It's been nearly 17 years since I had a military ID, so I'm not certain if that is/was right.
I have a current military ID (CAC) and it mentions nothing about what to do if found. Not something I’d lose though, what with name, birthdate, and social on it :/