Update: Owner found today, cat ("Carter") is back home.
Our neighbor showed up on our doorstep Wednesday night with a black cat her dog had chased up on a fence. We took him in planning to foster him until the owner turned up. Today we learned the owner moved out about about a week ago from a house down our alley. I have a Craigslist post up and I've notified the local shelter, so at this point we're just sort of waiting to see if she'll come around looking for her cat.
He is a very mellow and sweet guy, quite big, VERY soft. He looks like a black cat fashioned himself a vest out of a dark brown sheep.