A secret room is located on parking level 6. When you are starting to go down to parking level 5 you will see four gray barrels, jump on top of them. After that, jump onto the air vent, then jump to the roof. You should see a sort of a hut up there to your right. Shoot the light colored part of the hut and it should fall down. Go inside and you will fall through a vent in the floor. You can get ammo and other items.