When you approach the southernmost blue key-locked door, an alcove containing imps will open at the southeast of the map, in the green tech room. Head to that location, and press on the west wall of the alcove to find a backpack. (sector 255)
Once you step into the southern section of the gray marble room with the small nukage fountain, a few compartments containing enemies will open, including one southwest of the brown brick area you came from. Backtrack to that location, and open the rear wall of the compartment to find a megasphere. (sector 257)
After you climb the stairs into the hallway leading to the exit door, an alcove containing imps will open to the west in the previous room. Push on the rear wall of the alcove to find a heavy weapon dude. (sector 261) On Hurt Me Plenty or lower, there will be a blur artifact here as well.
In the tunnel with the exit switch, press on the middle of the northern wall to find a super shotgun and a medikit. (sector 259)