[throwaway98434]

Generally speaking theft requires an intent to deprive an owner of property. It's a crime to take my bike intending to deprive me of it, but it's not a crime to take my bike while attempting to return it to me.

Property owners do not have any obligation to lock up their items.

There are sometimes very specific laws regarding found property when the owner cannot be located, generally these involve turning the property over to the state first.