Boo Radar

The Boo Radar is one of the various features included in the Game Boy Horror in Luigi's Mansion and its 3DS remake. 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.

In Luigi's Mansion 3, Gooigi can vibrate when Luigi is nearby a door to the room where the Boo is located, the closer Luigi is to a Boo, the more Gooigi vibrates. This functions similarly to the Boo Radar.

Official profiles

 * Instruction booklet description (page 22): "This will appeaer during the game when Boos start to appear. When Luigi approaches a Boo, the lamp will turn red to let you know."
 * Instruction booklet description (page 28): "The Boo radar is a convenient function when you are wandering the mansion in search of Boos. When Luigi enters a room where there's a Boo, the lamp on the boo radar will change from blue to yellow. When Luigi approaches a Boo's hiding place in the room, the lamp will change color from yellow to red and start to blink. When this happens, examine all suspicious objects nearby. The Boo radar responds to all Boo-shaped objects of a similar size, though, so just because the lamp is flashing red does not mean you will find a Boo."

Names in other languages
Buu Huu-Radar Radar Scovaboo