Elevator Shaft (Basement)
From the Super Mario Wiki
The elevator shaft has several spiders and spiderwebs, as well as a ladder leading up to a Pearl. There is a painting Luigi can use the Dark-Light Device on to make coins appear. The blue Gem can also be found here.
After Luigi learns the Suction Shot, he can pull off the panel in the northeast corner of the garage to access the elevator shaft. He can climb the ladder to get higher up. Gooigi can then slip through the grate to drop down to the gem, which he can collect.