When you and your squad are inside that big building and trying to disable the "generators", you will eventually encounter a clock that is hanging on a wall above an archway. To find it, enter the building and go up the stairs to the right. Go underneath the archway ahead of you, turn around, then look up to find it. Equip your Gravity Gun and shoot it using Mouse 1. Keep shooting it until it is on the floor, with its face and hands facing the floor (so you cannot see them). Notice that it has no back. You will be able to see through it. You may lose it -- to find it, crouch on the floor and look for a semi-circular shape and shoot it with your Gravity Gun until you can see its face and hands again.