From the Super Mario Wiki
The Guest Room (Japanese: 来客用寝室 Guest Bedroom) can be found in Area Four of Luigi's Mansion. When Luigi enters the Guest Room, he fights and captures Sue Pea. After Luigi captures her, he exits the room and reenters it to claim a Treasure Chest.
The Guest Room contains quite a few objects of note. First, there is a vanity on the west wall that serves as a warp to the Foyer. The bed takes up much of the eastern half of the room, and there is a nightstand beside it. Behind the bed is a plant that can be watered for a Blue Jewel. It has a picture of a cat, along with a picture of two puppies.