From the Super Mario Wiki
This article is about the ghost enemy in Luigi's Mansion. For information about the rarer type of ambiance enemy in Luigi's Mansion: Dark Moon, see Mouse
A Gold Mouse
leaving its piece of cheese.
Gold Mice are apparitions of mice found in the game Luigi's Mansion. They come out when Luigi examines one of five pieces of cheese with his Game Boy Horror and then scurry around the room before escaping through the wall. Gold Mice also randomly appear in five predetermined rooms or hallways 20% of the time Luigi enters the room or hallway, for a total of ten mice in all throughout the mansion. If Luigi manages to suck one up before it escapes, a large amount of money will spray from the nozzle of the Poltergust 3000, which can include valuable treasures like Stones. Unlike with Speedy Spirits, if Luigi fails to catch a Gold Mouse, he is given other opportunities to do so, such as re-scanning a piece of cheese after exiting and re-entering a room.
This is a list of the locations of each Gold Mouse in the game and the rewards earned for capturing them.
Some of the Mice, though, are found in rooms (or hallways) without any cheese. When the player enters some specific rooms, there is a 20% chance of a Gold Mouse to appear.