Ghosts

 in the Call of Duty series refer to various in game events, modes of gameplay, or easter eggs put in by the developers.

Call of Duty: Finest Hour
In Call of Duty: Finest Hour, a ghost of a little boy can be seen in the Ghostly Room.

In the British second level, Depot Saboteurs, a small house can be accessed, looking similar to a church. Throw two sticky grenades at it, then press the Action button. Nothing will appear. Throw one more sticky grenade and when the explosion has cleared you will see a stairway. Go down it to find a strange room. Fuck Everbody.

In the room, there is the ghost of an Arab playing an instrument. He does not acknowledge you are there, just keeps on playing. A Bren LMG can be found, as well as a first aid kit and more sticky grenades. On the walls there are pictures. Some include SAS (Or perhaps PPA) soldiers wearing turbans in a Jeep, and one picture may be of T.E. Lawrence Of Arabia, but this in unconfirmed. The room has a mysterious lighting and a ghostly atmosphere. No one else will follow you down there.

Call of Duty 4: Modern Warfare
Appearing in Call of Duty 4: Modern Warfare, these are usually the cries of children that appear only in places of great destruction. In the level "Aftermath", when Sgt. Paul Jackson exits the downed helicopter, there is a playground to his upper left. If he were to walk over there he would find that the cries of children can be heard. Some people claim to actually see the ghosts walking around trying to use the things at the playground but it is unknown if there are ghosts there. In the level "All Ghillied Up", Lt. Price should be able to hear the same cries of children like before, but it's most likely from being told there were 50,000 people in the town at one point. However, if he were to kill the two men who are dumping bodies into the pond, while they are picking up a body, the programmed AI will still have the bodies get picked up and thrown into the pond, with no one there to do it.

Call of Duty: World at War
In Call of Duty: World at War, there are ghosts in the levels Der Riese, Shi No Numa and Verruckt. In Verruckt, disembodied cries and screams can be heard. In one room, there is a man screaming in agony, and in another room, a baby and a woman are crying. Also, if the chair in the left starting room is turned on (by holding X, square or F on PC) the sound of a drill and a man screaming can be heard afterwards. There's also writing on the walls in the room behind the Speed Cola. The ghosts cannot be seen, but heard. In Shi No Numa throughout the map some eerie whispering is heard. Also in Der Riese occasionally, standing near the power generator near the window children can be heard singing, and a woman can be heard crying near the furnace next to the Thompson. The same can be heard in the Fishing Hut in Shi No Numa.

That's not all, however, for there are multiple disembodied voices in many of the multiplayer maps as well. (Hangar, Dome, and Asylum, to name a few) Not only that, but in the singleplayer mission "Semper Fi", after the marine is executed by the booby trap and encounters his/her first "banzai charge", if they stand by the shrine beside the trap, the player can hear a voice speaking to them. This is best heard when the volume for the music is turned down entirely.

Call of Duty: Black Ops
In Call of Duty: Black Ops, the only level that seems to have Ghosts is the Nazi Zombies level Kino der Toten. Heading towards the Alley in this level and looking at the window that is on the left of the box spawn, the player will see a Door with a larger hole in it. Staying around the window a strange noise that sounds like whispering start to play. It is unknown who or what is making this sound.

After the players have teleported into the Pack-a-Punch room and then that gets teleported to a random room or if the player no-clips out of the map and into one of these rooms then the following can be heard. In Samantha's room (the room that's not destroyed) the player can hear her name being called and a strange child's voice (Most likely Sam's voice) can be heard at times as well. If the player teleports to the Dentist surgery, they can hear a drilling sound and a man screaming out. Due to the very small time amount you are in the random rooms it is often you will not hear some of these sounds. When a Hellhound round begins, after the Demonic Announcer says "Fetch me their souls," if no hounds have spawned yet, one can hear a quiet, and eerie whispering. If the player goes into the dressing room and head to the window that is past the MP5K and if the player stands next to if for a while, one will hear a knocking sound being made.

Also, in Shangri-La when riding the mine cart, one can hear children singing.

In the multiplayer map Villa, one can hear the sounds of men screaming and in the map Array, gunshots can be heard coming from different places in the map, even if the player is alone and has not fired their weapon.

Call of Duty: Modern Warfare 3
In Call of Duty: Modern Warfare 3, in map Sanctuary, player can hear a ghostly voices saying "Hehe, look at me, daddy! Hehe!", "Don't get too close to the road, sweetie", "Hehehehehe.", and "Sarah, don't go too far!" The voices are sound clips taken from Davis Family Vacation. This is only time when ghosts appear in Modern Warfare 3.