It is seen in the level where the player plays as Sam Rivers inside a tank. On the first time the player dismounts the tank and clears a building, there will be small farmhouse in front and to the slight left of the tank.
The smaller part of the wall should have a hay bale on it. Try and jump sideways into it, and after a few tries the player will jump straight through the wall, and into a secret room. The room has a few health-packs, which are rare in this certain level, and also has a giant chair in it. But the strangest thing is a small open door in a corner that looks as if it is doll-sized and the inside of it is glowing bright green.