Call of Duty: Finest Hour
In the mission: "Come out Fighting", if the player jumps on the wire barricade connected to the second house then walks to the wall, they will be able to pass through the wall and see a secret room where they'll be able to see a UFO.
The game has a hidden, blackened room filled with candles and the ghost of a young boy in a baby's crib. The boy doesn't seem to acknowledge that the player is there, and blankly stares into space the whole time.