Tanker chapter: Secret passage
In the Holds 2 room, go all the way to the left to find a hatch. Open it, and you will climb down into a small area. Crouch and go through the vent on the ground. You will eventually see a gun clip fall through and a man will start talking. His shadow will be on the ground. Wait there until his shadow leaves, then move through the area. After you get past that, you will reach an opening and an intermission sequence will begin. After it ends, crawl through a vent on the right side of the small room. After you crawl through, you will find a ladder. Climb up the ladder, and it will lead you to a small perch. Look down at the bottom of Metal Gear RAY, and you will see Ocelot standing there. He will then go away. You can keep doing this and experiment with things to do with Ocelot.