From the Super Mario Wiki
The Storage Room is a room in Area Two of Luigi's Mansion. When he first enters, Luigi is attacked by a Purple Puncher. After Luigi captures the ghost, the room lights up, though no key appears. Luigi presses two buttons in the room, opening a hatch that releases 50 Boos and King Boo. After the Boos disappear, Professor E. Gadd gives Luigi a call on his Game Boy Horror and tells him to come back to the lab.
The Storage Room contains a bucket, a stack of chairs, and several crates, all located in the eastern half of the room.
Ghosts in the Storage Room in the PAL Hidden Mansion
Ghosts in the Storage Room during the blackout
Names in other languages