In the outdoors area, walk under the second arch, and then head back into the building you came from to find that the floor has lowered to the south. Drop in and head behind the building to find security armor and a box of shells. (sector 61) Press on the pentagram that appears after you enter to trigger a lift back inside. Note that once you do this, you will not be able to return to this secret.
Due to the way in which the level's REJECT lump is composed, all of the Deaf Demons in sector 48 (the ground to and around the rock formation) are unable to see you, even if you are standing right in front of them, until they are attacked. Furthermore, all actors standing in this sector are immune to blast damage, which can be seen by giving yourself a rocket launcher using a cheat code, standing in front of a wall in this sector, and firing a rocket into the wall. Whether this was intentional or not is unknown.
In the red skull room, several plainly visible linedefs on the east side are marked as hidden for no apparent reason.