The difficulty will spike in this particular mission. Forget the previous one, especially if you've used heavy stealth to complete it. The silent alarm has been triggered (don't ask me how), and it's a rather short but very intense mission. First, there is a flak jacket in a box nearby if you need it. Go to the first crossing ahead and be ready to face enemies on your left. DO NOT TOUCH THE RED LASERS WHICH WILL KILL GABE INSTANTLY! For now, you can only shut the ones on your right by shooting the switch with a flashing red light behind them.
Ignore the green switch at this point and continue to the next lasers. Locate the red switch and shoot it. Notice that the lasers on the opposite site have been shut off. NOW you can come back to the switch with a green light and press Triangle to open the large door. The first two missiles you have to catalog are in there. An enemy is also waiting for you inside. Shot him and tag the missiles. Open the door on the other side, eliminate the enemy waiting behind it and keep going.
When you reach a double intersection then turn right at the first intersection and deactivate the lasers by shooting the switch ahead. A few enemies will try to surround you. Run towards the last switch you just fried and take your right. Again, turn of the lasers by shooting the nearby switch and keep going. You will come across a corridor on your right. There is a flak jacket behind the lasers. Ignore it otherwise. When you reach the next lasers, repeat the same process and open the door on your right (use the green light switch). There are two soldiers in the back of this room. Take them out and go to the control panel they were standing next to. Press Triangle to raise the four missiles and tag them. You can later come back all the way to the first of the double intersection.
At the second intersection, a bunch of enemies will jump on you. Deal with them and turn off the lasers on your left by shooting the switch. Like you did for the first storeroom, go deactivate the next lasers. Afterwards you can enter the room and tag the last four missiles (objective completed/checkpoint). Watch out as an enemy will attack you from behind as soon as you enter the room! When you are done, exit by the opposite door. Take your right at the intersection and destroy the last laser switch.
To the right of the control room is a freight-elevator. Use the switch on the wall to open its door and take it up to the roof (press Triangle). Mission complete.