Boo Radar

From the Super Mario Wiki
The Boo Radar

The Boo Radar is one of the various features included in the Game Boy Horror in Luigi's Mansion. After Luigi accidentally sets free all of the Boos in the mansion, E. Gadd shows it to him. The function of the Boo Radar is to detect the presence of hidden Boos in the different rooms Luigi enters (though it also detects other spherical objects, like Boo Balls and Bombs). If the light on the top of the Game Boy Horror is blue, there are no Boos in the room. If it is yellow, there is a Boo (or a Boo Ball or Bomb) in the room. If the light is red, a Boo, Boo Ball, or Bomb is very close to Luigi. When Luigi first enters the third floor balcony, the Boo Radar goes haywire anywhere Luigi stands in the area due to the presence of 15 Boos gathered in a single place.