Closed box at the northwest corner of level 7, Dead Simple:

There is a small rectangular room at the northwest corner
of the map which is only accessible during Deathmatch
games.  It contains a BFG-9000 and an Energy Cell Pack, and
an exit switch is mounted on its west wall.

A Deathmatch player who appears (or respawns) inside this
room can open its east-side wall.  Approaching that wall
from the inside will also cause the 4 interior walls of the
main courtyard to open up.

This room is not considered an official Secret area by the
game, so has no effect on the player's secrets-found score.