Furnace room, sealed door. No way for Investigators to get in, but bad guys can use it. Do I understand correctly?

Dan said:

That's right. Since it is the Sealed Door lock card monsters can ignore it.

Incorrect. The rules explicitly state that monsters CANNOT pass through a sealed door.

AstorSigma said:

The relevant rules are:

"Monsters and investigators may never move through a door covered by a sealed door marker." pg. 4

and

"Monsters may freely move through normal doors, and they ignore Lock cards." pg. 11

Since the rule about Sealed Doors specifies markers and monster movement states that monsters ignore lock cards, it doesn't matter what the name of the lock card is, and it does not add a Sealed Door marker to the board so the monsters can freely pass through it.

EDIT: Cleared up spelling mistakes and language.

*sigh* Valuable lesson learned: Don't ever call two different things by a "nearly" identical name. Or, better yet, if there are doors on your map which you don't want to use, call them "These-Are-No-Freakin-Doors" / "Replace-Door-By-Wall" / ... markers.

Sealed Door Marker = Used to make a door on the map tile unusable forever and always . Basically a tool for level design and level design *only*, the game regards these doors as "wall" and nothing more. They aren't useable. No. Never. By no one.

Sealed Door Lock Card = Regular lock card like every other lock card in the game. Can be bypassed by monsters, removed by events, possibly (!) even players if the story commands.