Did you find another Schiwnn?
I found a Raleigh Super Grand Prix last week it had no wheels so I deemed it was abandoned. Stripped it down. Just waiting for some spare cash to start building it up.

Is the bike ridable? Is it locked? If it's unridable (e.i. it needs wheels, handle bars or anythig else major I would say it's good to go. If there's no lock on it's also good to go. Let's face it the real owner would never leave his/her bike unlocked a thief on the other hand... This is where things get a little tricky, if you have and unridable bike that is locked up then I would say you have to wait at least 10 days. Give the owner some time to get some replacement parts or at least time to carry his/her bike home.