Red Coins are items in Luigi's Mansion 2. They only appear in secret ares and after completing a floor in ScareScraper. Unlike the typical coins in the Luigi's Mansion series, these coins are big, red, floating coins that do not count toward the treasure count.
In Story Mode, Red Coins will only appear in secret areas. When Luigi finds a secret area that is for Red Coins, there will be a small area as the room titled "???", usually in the form of some type of maze. To win the secret room, Luigi must collect all the Red Coins in the time given. Usually this is not hard, as the coins are given in a straightforward manner, however the time limit is a difficult part of these rooms. After collecting all the Red Coins, a chest will appear in the middle of the room, usually containing treasure, or sometimes gems. If Luigi fails, he will be teleported out of the room and back to where he was, and he cannot try again.