This is something I have wondered about myself. In Toronto, if they are locked to a bike post owned by the city, they will come and put a note on it to remove the bike or it will be cut off. Even in my building there is a bike locker with bikes that look like they haven't been ridden in years by looking at the dust, dirt and grime that has accumulated. I suspect that at least half are from ex tenants who never came back for their bikes.