From the Super Mario Wiki
The Laundry Room (Japanese: 洗濯部屋 Laundry Room) is a room in Area Two of Luigi's Mansion.
Using the Key obtained in the Fortune-Teller's Room, Luigi unlocks and enters the Laundry Room. After he enters, he sees a door on the the front wall and one on the back. He burns the door on the back wall with his Fire Element, revealing the door to be fake and causing the ghosts to attack. After the ghosts are captured, Luigi inspects the room and discovers Mario's Cap in the washing machine.
The Laundry Room is a small, cramped room. Nonetheless, it has space for a washing machine, a laundry basket which contains a heart, a cabinet, and a small bucket complete with mop and cleaning supplies on a high shelf.
 Ghosts in The Laundry Room in the PAL Hidden Mansion
 Ghosts in The Laundry Room during the blackout