From the Super Mario Wiki, the Mario encyclopedia
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A trapdoor (also known as a fake door) is a special trap that resembles a real door encountered in the Luigi's Mansion series. They are designed to confuse and injure Luigi as he progresses through the games.
In Luigi's Mansion, trapdoors are found in various locations on different floors of the mansion. If Luigi happens to turn the knob of a trapdoor, the door springs open and flattens him against the wall, causing him to lose ten HP and a few coins. There is a glitch that allows Luigi to get smashed unscathed, but only a few times before taking damage again as normal. A ghost can be heard laughing after Luigi is smashed. Sometimes, a trail of conveniently placed coins leads straight to a Trapdoor to catch Luigi off guard.
The only way to defeat a trapdoor is to set it on fire; while real doors don't catch on fire, Trapdoors go up in flames and disappear. However, if Luigi enters two different rooms and returns to where the Trapdoor was, it will have reappeared. During the blackout in the game, the ghosts increase the amount of Trapdoors in the mansion and replace certain real doors with Trapdoors, including those leading to the Storage Room, the Projection Room, the Kitchen, the Parlor, and the Anteroom.
There are several ways to tell a Trapdoor apart from a real door. For instance, Luigi glances at Trapdoors as he walks by them, but doesn't look at real doors. Additionally, Luigi can check his Game Boy Horror; real doors appear on the map, while Trapdoors don't. Luigi can also vacuum a door with the Poltergust 3000; if it rattles, it's real, if it doesn't, it's a Trapdoor. Another way to differentiate between the two involves looking for doormats; real doors have doormats under them while Trapdoors do not (though in some cases, a real door, like the one leading to the Fortune-Teller's Room, won't have a doormat either).
Luigi's Mansion: Dark Moon
In Luigi's Mansion: Dark Moon, Trapdoors are found within Gloomy Manor, Haunted Towers, Treacherous Mansion, and the ScareScraper. They serve the same function as they did in the first game, though Luigi now loses only five HP when hit by one (two HP in the ScareScraper), and does not lose any Coins. Additionally, ghosts are no longer heard laughing after Luigi gets flattened by a Trapdoor, barring one instance in Treacherous Mansion when a Greenie appears to laugh at him. Just like in the first game, Luigi can use his map — this time on the Dual Scream — to determine whether a door is real or not, as Trapdoors will not show up.
Luigi's Mansion 3
Trapdoors reappear in Luigi's Mansion 3, this time exclusive to ScareScraper mode. When they trap Luigi, they do not close, forcing another player to use the Suction Shot to destroy it. They do not appear on the map nor do they have doormats. They can be destroyed by the Suction Shot to avoid triggering them.