From the Super Mario Wiki
The Sitting Room (Japanese: 来客室 Guest Room) is a location that can be found in Area Four of Luigi's Mansion. It does not need to be cleared to finish the game.
As soon as Luigi sets foot inside the Sitting Room, the door seals behind him. Luigi uses a Fire Elemental Ghost to light some sconces on the north wall of the room. This causes five Gold Ghosts and two Temper Terrors to appear. Once Luigi captures the ghosts, the doors are unsealed.
In the center of the Sitting Room is a table with an easy chair and a sofa around it. The table has an always-burning candle on it, a source of Fire Elemental Ghosts. The southwest corner holds a vase on a small drawer, a source of Water Elemental Ghosts. A plant in the northwest corner can be watered for a Gold Bar.
Ghosts in the Sitting Room in the PAL Hidden Mansion
Ghosts in the Sitting Room