Here is my Doom Episode 1 Secrets Guide. I have compiled all of the Doom secrets in a handy walkthrough with descriptions, pictures, and a video of each map, so you can easily find them all.
E1, M1: Hangar
Scroll to the end to watch a video walkthrough of this map that displays each secret area.
Secret 1
Right after you get through the zig-zag pathway over the slime, there is a discolored, brownish section of wall to your right. Open it, and the secret passage will lead to the blue armor in the outdoor courtyard area.
Secret 2
In front of the exit door, is an area lit by flashing lights. You will hear a platform lower or click when you enter that lighted area. This triggers the platform where the imps were in the zig-zag room to lower, revealing a shotgun and ammo.
Secret 3
Go back to the hallway that enters into the zig-zag room. Stepping onto the space where the lighting changes from dark to light will activate an elevator-like platform to temporarily lower inside the hidden shotgun area (Secret 2).
Run quickly to catch the platform before it raises up. If you miss it, you can always try again.
The platform leads to a dark walkway with quite a few health and armor bonuses!
Please continue to the next pages to see more Doom Episode 1 secrets, including a whole secret level!