What should you do if you find a bike that appears to be stolen?
As private citizen you cannot do much more than reporting it to the police.

If it was stolen and the theft reported, the legitimate owner may have it back.

If it was thrown away, there is the chance that the police will arrange the removal.

(Both points strongly depends on the country where you are located. In some places police officers may simply laugh back at you for reporting a suspected stolen bike, but again, if you take it you are breaking the law, too)