What does "unclaimed" mean?
In this case, "unclaimed" is an adjective used to describe things that do not have an owner - things that have not yet been claimed or possessed by someone. Thus, we can understand "unclaimed items from the lost and found" to mean items from the lost and found that have not been returned to/claimed by an owner yet. Ex: There are many unclaimed items in the subway lost and found. Ex: The unclaimed island did not have any human inhabitants.