Elite Guard #1: In plain sight, lying against some crates behind the blue door that requires the blue keycard.
Elite Guard #2: At the very end of the mission, right before riding the huge platform elevator to the VEGA core, move around the platform (check your map) and drop down below. Hop up the walkway to find this Elite Guard.
Classic Map: The lever for this map can be found directly behind the crates the first Elite Guard is lying against. Move back to the area where you found the blue keycard to spot this map.
Quakeguy Collectible: At the beginning of the mission, outside, drop down and go over the pipes leading to a maintenance door. This collectible can be found in the shaft behind it.
Jungleguy: After dropping down the hole to the left of the first Elite Guard, turn right to the 'dead end' and look down to spot a lower walkway. This collectible is at the end of that lower walkway.