After a bit of a rough landing, Luigi found himself in the Hidden Room. This room was full of treasure chests, and the plumber, in a greedy Wario-esque moment, began opening them. The plumber was caught off guard, however, when he discovered a Flash ghost inside one of the chests. Using the Fire Element function on his Poltergust 3000, Luigi was able to defeat the apparition. Luigi hoped that was the only ghost in the room. No such luck, however, as the ghost-hunter would have to capture three more Flash ghosts before the room brightened. When it did, Luigi noticed three chests still unopened. He raided them all, and inside one of them was a Blue Jewel, a Green Jewel and a Red Jewel. Luigi rejoiced at his good fortune, and after celebrating, he used the same mousehole to transport him back into the Butler's Room.
The Hidden Room is a small room with many treasure chests. Eight of them to be exact, including the one Luigi receives for brightening the room. The room's floor and walls are golden and as such the entire room sparkles with gold dust, like the Sealed Room. During the Blackout, one can come back here and capture a Speedy Spirit. Two vases are also located in the room (which are worth checking), as well as a mirror on the south wall that can be seen with the Game Boy Horror.
In the PAL Hidden Mansion, Luigi will find Shy Guy Ghosts instead of Flashes.