Taking a bike that has been left unlocked without the owner's permission is generally considered theft. Even if a bike is left unsecured, it does not justify taking it without the owner's consent. In most jurisdictions, the law considers the act of taking someone else's property without authorization as theft or larceny.
It's essential to respect the property rights of others and abide by the law. If you come across an unlocked bike that doesn't belong to you, it is best to leave it as you found it and not take it. If you believe a bike has been abandoned or is posing a safety hazard, it is advisable to contact the local authorities or relevant organizations responsible for managing abandoned bicycles in your area.