I park my bike at Amsterdam Central Station every morning, it’s pretty crowded and there are definitely some bikes that have been parked there for a while. A couple of days I go I noticed that a number of bikes have been tagged with bright orange waarschuwing – warning notices.
If your bike is tagged and you don’t move it by 7 September, the gemeente (city council) will remove it for you. It then gets stored in a warehouse somewhere hard to get to, and you have to go and liberate it.
Bad luck if any of the bike owners are on holiday at the moment.