Once you leave the chair and got your camera back, hide next to the door were Trager comes trough. Then run for the vent and open the door leading back to the elevator. Then make Trager chase you, close the door and go back trough the same vent. Next make a run for the next door. It's possible he spots you while opening that door but normally you have more then enough time to open it. Then run for the next vent and get the key. Then go back and hide next to the door. Wait untill he walks into the room and passes the pillars, then make a run for the elevator.
