When i'm not cycling to work, I walk in. There are quite a few bike racks that I pass when walking to work, and every few months the council goes around checking the bike racks and marking bikes that may have been abandoned.
I once spotted a good looking Kona bike that would have been good for a friend of mine that was tagged up, so i went up calling the council (H&F) to see if there was a legal means to getting the bike after the period of notice was up. It turns out that due to H&S concerns, they won't pass the bike on to the public and instead landfill them.
This, to me, seems to be a wasted opportunity. These bikes would be great for a community project in taking these bikes, doing them up and reselling on cycle to work schemes or similar.