Mission: The Ark
Same mission. Defeat the Scarab and enter the interior structure. Fight through the first few rooms, including the room with the sleeping Grunts and Brute, until you hit a narrow, downward ramping hallway. Through the door at the end, you'll find a larger room with a wider and shorter downward ramp. Below that ramp is a door hiding the third terminal.
In this case, Guilty Spark will actually lead you to the right door. Just watch him.