“Hold on,” she said, pressing an ear to the door and listening intently. If she heard nothing of suspicion—that is, nothing that made it sound like there was something moving behind the door—she’d knock on it, then listen to the tone it created. If the sound was light and travelled, it meant the door wasn’t blocked. If the sound didn’t travel, there was probably something blocking the door, and quite a big something too.
Thought 1, rolled 6
Pebs listens close, and in the silence of the room, she is able to hear... nothing. If there's anyone behind the door, they're either inhumanly quiet or not alive. The only thing she is able to hear the soft hum of the air conditioner from the vent above.
Knocking on the door reveals that it is one of those cheap, hollow doors made of thin plywood reinforced by a lattice of cardboard to make it feel solid. She once went down a rabbit hole while looking at online encyclopedias and ended up researching doors, and she's confident this is a hollow-core door. They're tough enough, but mainly only serve as ways to separate rooms in houses.