After you have came through the fan and reach the first checkpoint, an engineer will walk past. After he has left, run and jump over the fence. Knock him out, then hide him in the dark. Do not run round the corner, as there is a camera at the other end. Carefully sneak past it and you will reach a set of stairs. Go up the stairs, but be cautious as there are two guards up there. Go into the first room on your right. Pick up the items and go through the opposite door to bypass half the level.