For example, in many cities once every week every bike parked in a public area is labelled with a little color ribbon. The colors rotate according to a fixed schedule, so that it can be verified if a bike has been parked for more than two, three, four weeks. If a bike has been left unattended for more than N weeks (where N can vary) it gets towed away. They all get stored someplace out of town, and once a month the bikes that haven't been reclaimed for M months get sold for cheap.
In other words, there are systems for how long you can leave your property unattended in public before it ceases to be your property.