In the northern part of the map, in the cavern's south, southeast of the platform you enter on, there is a corner with an imp and a low ceiling bearing a square grid of red lights. Firing any hitscanweapon beneath these will reveal a button. Pressing it will cause the extended wall on the walkway to drop (to reveal a super shotgun on Ultra-Violence or higher). Beyond it is a passage (sector 66) with a heavy weapon dude, a medikit and a box of shotgun shells.
In the passage with the rocket launcher (south of where the red key is found), a mangled corpse is chained to the wall. You can open this wall like a door and find a box of rockets (sector 149) between a health bonus and an armor bonus. Beware, however; a pair of heavy weapon dudes await on either side.