Be aware. If the owner reports it stolen, you get charged when they catch you with it riding. Despite the fact that it was 'abandened', it can' be reported stolen.
Legally I think you have to report it as found.
I'd probably just keep it anyway too, but officially, I don't want you to get into trouble for picking up a bike that probably fell off someone's vehicle and won't ever be looked for.
Nice pics!
Wolfy