A monster closet is a scare tactic used in Doom 3. They are generally walled off corners that will suddenly open, depending on the player's actions (i.e. if they approach it), to reveal an enemy. Sometimes they can be well hidden, but a keen player would notice certain walls that are commonly textured as doors. Zombies and Imps are generally the most common enemies that will utilize monster closets to attack the player. However, there could be occasions other enemies like the Z-Sec or the Revenant will also utilize this tactic.
Many of these closets often have powerups directly next to them, giving the player an incentive to move themselves towards an otherwise dangerous trap. Monster closets are very common in the first half of the game. But as the player goes through later levels, they gradually decrease.