Elite Guard #1: In the area with the Hell Knight hologram, follow it and hop down. The Elite Guard can be found in the nearby hall.
Elite Guard #2: In the manufacturing hall, right before entering the area of the Gore Nest, go up the crates and into a hidden hall up ahead. The Elite Guard is found at the end.
Vaultguy: In the area where you found the yellow keycard on the body, drop down on the side (be careful you don't fall into the hot metal). Vaultguy can be found here.
Prototypeguy: In the manufacturing area, check in one of the carts for this collectible.
Classic Map: On the long connecting walkway in the main area, look to the side for a lever. In the area up ahead (where you can also find an Argent Cell in the corner), this map becomes accessible.