A Room Zero is a room that doesn't appear on public blueprints (that's my understanding of it anyway). My first encounter with a Room Zero was in the Abandoned by Disney trilogy by Slimebeast, my favorite creepypasta. The idea of a room that "doesn't exist" fascinates me and I often use it in a few stories and projects. I write them as rooms that hold things that shouldn't get out or something only a few people have the access to see and interact with.

I don't know what's in this Room Zero, but it must have been something dangerous if the door was supposed to stay locked....