AKA: It Means A Room That Only Has Access Via A Trapdoor, Okay?
Mulder's sister having been abducted is a huge part of his character, but it's one that doesn't seem to come up much. That's not a complaint, nothing is more irritating than a character who feels the need to spout off their character traits every few scenes, but it's just something that winds up totally downplayed. That's healthy for the show, since it means the moments where it comes out feel natural and real, and it often functions as background motivation for some of Mulder's obsessions.