At the east of the room where you start the level, there is a hole in the south wall, beyond which you can see exposed circuitry. Shoot it, and it will quickly fall open, allowing you to get a backpack. (sector 194)
After the northern door at the beginning, the westernmost passage to the north has a demon-faced wall with a candle in front of it. Open it to find another backpack and a Pain Elemental. (sector 513) Note that you can only open this door once from the outside, meaning that you will only be able to access this secret once.
At the very north of the map, press on the bloodfall, and it will lower to reveal a rocket and some shells. (sector 511)
At the east of secret #3, there is a hallway with two demonic faces, one of which has a light shining near it. Press on this face to find combat armor. (sector 141)
In the room just southeast of secret #4, there is a bloodfall in the northeast with five armor bonuses sitting on top of it. Press on this bloodfall, and it will lower to reveal 13 more armor bonuses. (sector 368)
At the end of the hallway where you find the yellow card is a scrolling wall with faces on it. Open it to find a rocket launcher. (sector 486)