The Hidden Room (Japanese: 隠し部屋 Secret Room) is an optional room found in Area Two of Luigi's Mansion. It can be accessed by inspecting the mouse hole in the Butler's Room with the Game Boy Horror, activating a warp that will suck Luigi through said hole.
Once Luigi arrives in the Hidden Room, he sees several treasure chests and begins to open them. One of the chests has a Flash ghost inside, so Luigi defeats it with a Fire Element. After Luigi captures three more Flash ghosts, the room brightens, and Luigi can open the three remaining chests to obtain a Blue Jewel, a Green Jewel and a Red Jewel.
The Hidden Room is a small room with eight treasure chests, 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, Luigi can come back here and capture a Speedy Spirit. Two vases are also located in the room, as well as a mirror on the south wall that can be seen with the Game Boy Horror.
In the PAL Hidden Mansion, Luigi finds Shy Guy Ghosts instead of Flashes.
Ghosts in The Hidden Room during the blackout