Pokémon Mystery Dungeon: Red Rescue Team. It's an item that unlocks doors that appear in random locations in a dungeon. One example is in Solar Cave, used to find a link cable on floor 10 and an HM on floor 15. They are used for many other doors as well. Keys are often found in the Desert Region, but can be found on the ground of many other post-game dungeons as well. In Pokémon Mystery Dungeon 2, keys are usually found in the Zero Isle regions and Aegis Cave.