Valve headquarters is full of secrets, and there's a constant stream of spies trying to infiltrate the inner sanctum. This is why Valve have splashed out on an extra bit of security for their lobby, a life size sentry gun from Team Fortress 2, complete with motion sensors. There's a video below of the sentry gun in action.
The sentry gun was built by the WETA workshop , a design and manufacturing company that has created props for the Lord of the Rings films and Avatar. Now they've turned their attention to Team Fortress 2, and created the remarkable machine shown in the video below.