Thief hideouts are locations scattered throughout Hyrule where the player may find various treasures. Most of them are abandoned and now roamed by Moblins. They each have a main room with a door tightly shut by a lock and key. Unfortunately for the player, these locks are designed to engage their lock and then have the keyhole removed so it may be hidden in addition to the key. Both are hidden deep within the maze of the hideout.
These are typically hidden underground or behind a wall and require an explosion (or the Earth Cog, in the case of walls) to reveal them. The player will know when they've come across one by the texture of the ground.