It depends on the jurisdiction, but a lot of places have laws around found property... often a finder would have rights to truly abandoned property, but there are usually laws in place that establish a procedure for determining if something is truly abandoned or just lost/misplaced/stolen.
Usually that process involves turning the property in to the government, and waiting some period of time for the original owner to claim it. If it is not claimed, it would become yours.
I don't know of any law that requires property on public space to be locked. There may be laws saying you can't leave property in public for more than a certain amount of time, but I am pretty sure none of those would allow anyone to just take the property.