Lost and Found
From the Super Mario Wiki
The Lost and Found is a location in Luigi's Mansion 3. It is found on the second floor of the Hotel Shops on the third floor of The Last Resort. It is an area connected to the Security Office. Kruller is fought here.
After Luigi gets the star key needed to unlock the door, he can enter the security office to find Kruller sitting at his desk. He retreats to the lost and found area and grabs a water pistol which he uses to defend himself. Due to there being bars between the security office and the lost and found, Gooigi must fight Kruller. This also means only Gooigi can explore the lost and found area. After defeating Kruller, Gooigi gets the second floor elevator button.
There are several boxes in the lost and found, some of which contain oranges, cherries, or newspapers. There are also shelves holding lost items.