lol i don't know where you live but a complete working bike with air sitting by itself unattended would not last 20 minutes here. I put out a completely stripped women's MTB frame on the sidewalk and it was gone within an hour.

A complete working bike might have been someone's though, so I think you would be obligated to wait around for a little while to make sure. Probably should report it to the police too to make sure you don't get busted riding stolen property.. it could be stolen and abandoned.

but yes if you see a free bike, take it, cause someone else will if you don't. especially if it's working.. it's a free ride somewhere for someone at the very least