The secret exit for Scourge of Armagon Episode 1 is found in HIP1M2: Storage Facility. This is a written guide + images to find the secret exit, a video clip is also provided at the bottom if you prefer.
As your progress through the level you will find the gold key on a raised walkway in a room full of crates. From here follow the walkway to a hallway that leads to a new weapon for this episode, the Proximity Launcher.
Grab the new weapon and immediately turn left to the panel as it opens. You'll notice a red target button on the wall ahead. A few enemies have spawned below so take them out, then launch a proximity mine so it lands on the button.
Now run down the hallway directly below the button. Follow it along the force bridge that you activated earlier and keep going right around until you reach the edge where the silver key was. There's a door across you can't reach, just wait a bit and eventually the mine will explode, trigger the button, and activate a bridge to the door.
Quickly run across and down the hallway to find the secret exit.