Some more details might be helpful, like:
Where? When?
Can you post a photo?
In any case, when you "find" a bike you are supposed to report it to the Police for two reasons. 1. So that they can (or are supposed to) try and re-unite it with it's owner (who might have reported it stolen), and, 2. To cover your own backside as "theft by finding" is a criminal act (in Scotland anyway; I'm sure they will have similar in the rest of the UK if that's where you are located).