When this sort of thing happens to me, I pick the bike up and turn it in to the Police. If the bike goes unclaimed, I get the bike three months later with a record to prove ownership. Keep in mind that if the original owner sees you with his bike, you could be charged with possession of stolen property.
Just taking it seems a lot like stealing to me. I might add that this dilemma has presented itself in other threads in this forum and sparks all kinds of opinions. This was simply mine.