Storage Room (Mezzanine)
From the Super Mario Wiki
The Storage Room is a location in Luigi's Mansion 3. It is found in the Mezzanine on the second floor of The Last Resort. It is a storage room west of the Dressing Room. The Dark-Light Device is obtained here.
When Luigi enters the Ballroom, he sees Professor E. Gadd trapped in a painting. He must follow Polterpup to the storage room. The door is locked, so he first needs to get the key from the dressing room. There is a safe inside the storage room Luigi must flash with the Strobulb to get the Dark-Light Device. Polterpup shows him how to use it on a painting of coins. Luigi is then able to use the Dark-Light Device to free the professor from his painting.
There are several draped objects in the storage room. One is a stack of boxes with mice. Another is a shelf of items with coins and a piggy bank hidden within. Another is a chair with Gold Bars. Behind the chair is a treasure chest with money inside. To the left is an area with a few spiders and an invisible treasure chest in the northwest corner. To reveal the treasure chest, Luigi needs to use his Dark-Light Device.