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The Mansion Safe is a scrap found in Paper Mario: Sticker Star. Mario is required to retrieve it in The Enigmansion by traveling to the living room to the left of the mansion's front entrance and Hammering a glowing portrait above the fireplace.
Mario must then travel through the remote corridor to the bookcases and enter the door, then remove the Mansion Bookcase and place the Mansion Safe into the resulting blank area, allowing him to battle the Gold Boo.