On the Underground Passage level, you'll run across a door that can't be opened, and, by "can't," we mean "you have to throw two grenades at it, press the action button, and then throw another grenade to blow it up." This will lead you to a circular room with candles floating in the air, baby pictures on the walls, a hog-size rat hidden under the stairs, and a tiny tank rolling around on the floor. It all plays out like a scene from some kind of fucked-up satanic movie, such as Harry Potter And The Chamber Of Secrets or something.
Oh, and don't forget to go up to the crib in the middle of the room, because otherwise you'll miss the ghost child staring at you.
This being COD, he speaks exclusively in racial slurs.
There are actually a whole bunch of ghost rooms hidden throughout the single-player campaign -- another features a turbaned man playing the flute in front of (we're assuming) a recently used hookah, surrounded by pictures of soldiers on the walls. After a couple seconds, the man starts to fade away, and you realize it's a literal Call Of Duty: Ghost.
"And after aaaaaall ... you're my wonderwaaaaaall ..."