Instructions
Activate Nuketown
Start a match with as many players as possible. Advertise that you are not looking for a traditional match, but rather one to find the Easter egg.
Locate all of the mannequins throughout the game. There are mannequins scattered throughout the houses and backyards in the game.
Race to shoot off all of the mannequins' heads as quickly as possible. Both teams must participate.
Listen to the music from the loudspeakers. If the teams shot off all of the mannequins' heads quickly enough, "Sympathy for the Devil" by The Rolling Stones starts to play.
Hidden Room
Go into the yellow house with the broken windows.
Walk up to the stairs to the top floor. Jump until you are standing on the door frame between the bedroom with the bunk bed and the room with the mannequin.
Turn left and face the wall with the cabinets. You should see a space in the wall between the two rooms.
Walk through the space in the wall until you reach the next room. The room has a mannequin and a great vantage point for sniping.