While legally the film may belong to the buyer, I suspect copyright stays with the original photographer.

Ethically, if I had contact info for the owner, I'd return it undeveloped.

As a coincidence I happen to have a roll of undeveloped film, from a prior owner, now.

I discovered the film in a camera that I have owned for over 25 years. (I can procrastinate with the best of them.)

I probably purchased the camera at a flea market in the early to mid 80's. While curious as to what might be on there, and whether or not the old B&W film still holds anything, I think I will just stick the exposed band on it, and leave it for display that way.