Start by going to the western wing of the base. Go to the fork (M8,5) to the north and climb down (B-2). Walk over into the next corridor, go left on the fork and then right to walk into the room with the safe. In there, you will find two guards and the safe (M8,6). You can either open it with the combination you have found on the officer (see above chapter) or with explosives. Apart from documents, you will find some ammo inside (including suppressed ammo for the rifle).
If you notice on the radar that you have been heard and enemy units approach climb downstairs and wait it out. Alternatively, equip the rifle and eliminate them, which makes a good solution also in this mission.
Climb down the stairs to a lower level. There are three guards here - also they can be eliminated with the rifle. Take interest in the big tank in the corner with a valve (M8,7). Turn the valve to damage the tank. Then, return up to the first fork over stairs.