From the Super Mario Wiki
The Service Hallway is a location in Luigi's Mansion 3. It is found in the Basement of The Last Resort. It is a long hallway east of the Laundry Room, south of the Garage, and west of the Service Stairwell. This is the second area of the basement to be explored in the game.
There is one coin inside the trash can left of the door to the garage. There is a painting to the right of the garage door Luigi can later use the Dark-Light Device on to get the purple gem. There are coins and Gold Bars on pipes above Luigi can use the Poltergust G-00 to pull down. There is also a fire extinguisher with coins inside.
Further to the right is the elevator. There are also bins and cabinets which contain some money. The potted plant in the northeast corner has Bills hidden with its leaves.
Later in the game, Luigi must escort Professor E. Gadd through the Service Hallway. While he is doing so, a painting falls off the wall and breaks, startling Luigi and causing the professor to laugh at him.
After rescuing Toad from the Boilerworks, Luigi needs to bring him through the Service Hallway. While doing so, a Slinker attempts to pull Toad in through a floor portal, while Mini Goobs emerge from a painting.