“Search fro the source of the security breach.”
Head back toward the entrance of this bunker room. You will see 2 vents. The left one is open while the right-hand one still has its grating. Shoot off the grating on the right-hand one and head inside.
There is an opening in the floor as you walk through the vent. Drop down into it to move forward. Head over to the right and then go through the hallway on the left.