One of the hidden collectibles in Luigi’s Mansion 3 is hidden Boos. After you’ve progressed a few hours into the main campaign, E.Gadd will inform you that you can start hunting Boos on each floor of the hotel. The Boos aren’t in the same location for each person, so you’ll basically need to use a couple of tools to track them down.
Boos won’t be in the same spot for everyone
What you’ll want to do to catch the Boos in each floor is to feel the rumble in the JoyCon and then interact with items when the rumbling is strongest. This will reveal the Boo to you, and then you must start the second phase of the battle to catch the Boo.
Catching them requires that you shine the Dark Light on them to weaken them and then use Suction to snag them and then throw them around like your standard ghost. Each Boo should take about two catches before you can put them down for good. You can also check the Boos that you’ve collected in E.Gadd’s Lab in the Parking Garage.