To get to the hold that houses Metal Gear RAY in the Tanker chapter without being noticed, first knock out the Marine by the projector in Hold 1, then get on your belly and crawl to the left side of the hold. Once you reach the left side, stand near the bulkhead and press Triangle to open up the maintenance hatch on the floor; the hatch will reveal a ladder and a secret tunnel. Climb down and go through the tunnel, it will lead you to the door to Hold 2. Once inside Hold 2, go to the right side of the hold and open up the maintenance hatch on the floor there. It too will open to reveal a secret tunnel. Crawl through the tunnel until you reach Hold 3, a movie of Snake looking at Metal Gear will play. After the movie is done (skip by pressing X) run over to the right side of the room Snake is in. Crawl through the vent, through the tunnel until you reach a ladder which leads to a ledge inside the Hold. Slide down the pole by pressing Triangle, and you're in!