A piece of this outfit will be added to the reward when a preening ibis card is used in the treasure room. When a player already has the full set and uses a preening ibis card, he or she will not receive a piece of the outfit, but a hoardstalker ring instead. So in that case they may want to leave any preening ibis cards on the floor, so other players can claim the items. This will also prevent them from using cards to steal, which lessens the chance of others stealing your reward, should it be better than theirs.