From the Super Mario Wiki
The Service Stairwell is a location in Luigi's Mansion 3. It is found in the Basement of The Last Resort. It is a stairwell east of the Service Hallway and southeast of the Garage. The Service Stairwell is made up of a flight of stairs leading up to the first floor, as well as a vending machine and a hidden treasure chest. A large statue of Hellen Gravely with a feather boa can be seen at the top. The yellow Gem can also be found here.
To explore the Service Stairwell, Luigi first needs to get the Poltergust G-00 from a car in the garage, then use the Strobulb to flash the light which unlocks the door. He then needs to follow Polterpup up the stairs and use a Burst to clear away the garbage can blocking the way. He is then able to access the first floor of The Last Resort, the Grand Lobby.
If Luigi flashes the green light on the vending machine, coins will come out along with some food. There are Bills stacked on the table to the right of the vending machine, near the bin. In the bin itself is some treasure. There is also a treasure chest under the stairs, hidden by cobwebs. There are more coins in the fire extinguisher.
After learning the Suction Shot, Luigi can pull down the vending machine to reveal a secret compartment with a couple of Gold Spiders and a painting he can use the Dark-Light Device on to make coins appear.