Fake Door

Are you looking for the Door Mimic?

Trapdoors (called "Fake Doors" in the Strategy Guide) are doors that look like the real doors from Luigi's Mansion.

Strategies
It's a good idea to defeat the Trapdoor by using fire and the fire will destroy it.

If Luigi opens the Trapdoor, he gets slammed in and Luigi hears a ghost giggling.

As Mice and Bats are about to attack Luigi when behind it, Luigi must burn or vacuum these annoying undead mammals then burn the Trapdoor.

Luigi must not underestimate his flame element ammo. He can get more from the room with invisible ghosts.

When Danglers, behind it come out and one tosses a bomb, Luigi must avoid the bomb, let them disappear or vacuum them then burn it.

Trivia
If Luigi vacuums the Trapdoor, the Trapdoor never shakes as the real ones do.

The Ice and Water elements never work against this type of enemy.