I can't imagine bikes being left unattended for the periods mentioned in this thread -it wouldn't happen here (South Brum) - for example
I got chatting over the fence to a tenant of a house whose garden backs onto ours. He dropped it out that he'd just been convicted of bike theft. Saw the bike at the side of the road, apparently, didn't know whose it was so he took it. When I commented that maybe he didn't know who it belonged to, but he knew who it didn't belong to, he thought for a bit then replied - "Oh, I see you don' t agree with that sort of thing then".
Leave it in the front garden and one of the local scrappies will drive by and pick it up for you - they have an unwritten rule, apparently, that if 'owt metal is left on the front garden within a few inches of the boundary it's "fair game". I politely disagreed, and for some reason he became quite threatening. Don't understand it at all.
So - no problem with abandoned bikes in our part of the world, they do get removed quite promptly