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The Boo Radar is one of the various features included on the Game Boy Horror. During the events of Luigi's Mansion, after Luigi accidentally let free all of the Boos in the mansion, E. Gadd showed him the Boo Radar. If the light on the top of the GameBoy Horror is blue, there are no Boos in the room. If it is yellow, there is a Boo in the room (or a Boo-related item, like a Boo Ball or a Bomb). If the light is red, a boo or a related item is right is very near Luigi.