All adventure sites have an icon graphic with the items that get dropped there. If there's an infinity sign, it's a common item that you can get without limit. If there's a number, then you get exactly that many of the item and no more, and that icon will disappear afterward. Those are the rare/legendary/unique treasures it refers to, and the treasure finder increases the drop probability of all of them. Eventually you will clean them all out and the finder will no longer do anything special there.