This is a screenshot of the area you are looking for: (Click on any thumbnail to enlarge the image)
First you should check to see if the TRAP is armed. If this laser is active, then the trap is armed and you need to turn it off. Turn it off by shooting the switch as shown in the pictures:
Drop down onto the ventilation shaft and shoot the grill. It will slide down and reveal a lift platform. Crawl onto it, and stay crouched while you ride up to the top. Crawl along the shaft, grabbing the cells, power shield, +100 health and backpack and simply crawl off the edge. A delayed door will drop you onto a pile of boxes below.
Yeah - the exit is pretty well hidden...