From the Super Mario Wiki
The Smuggler's Hideout is a room inside of the Secret Mine in Luigi's Mansion: Dark Moon. The Smuggler's Hideout is accessed by taking the door on the bottom of Skip Slope. The Smuggler's Hideout is a small room with a bonfire in the middle. There are several Treasure Chests around the room and a painting with a coin on it. There are two suits of armor on the right of the room and a window towards the north wall. The door on the left leads back to the Skip Slope while the door to the north leads into the Shaft.
During D-2: Hit Rock Bottom, Luigi needs to reach the Smuggler's Hideout to reach the Shaft in the other room. When Luigi enters the room, the door to the Shaft is blocked off by a huge chunk of ice. When Luigi tries to melt it using a burning log, a Sneaker opens the window nearby, putting the fire out and allowing two Greenies in, one of which has a paranormal shield. Once these three enemies are taken care of, Luigi must close the window so he can be able to melt the ice. Melting the hands on the suit of armor with a spear allows it to take off the helmet of the other suit of armor, revealing three gold bats. In D-3: Across the Chasm, the Strong Greenie hides inside the Smuggler's Hideout. The Strong Greenie hides inside a suit of armor. Luigi must use the Dark-Light Device to reveal a missing suit of armor that takes the helmet off of the one the Strong Greenie is hiding in. Before Luigi can fight the Strong Greenie, he must fight three Sneakers that the Greenie brings in by opening the window. After capturing the Strong Greenie, the green chain is broken.