Why can one other person make an empty public subway hallway feel safer but a private bedroom feel more unsettling?

Answer: A person fits public norms and breaks private expectations.

A person always triggers alertness because strangers can threaten. — Plausible, because an unexpected stranger in a bedroom can signal possible intrusion. But if strangers always triggered alarm, the same person would not calm the public hallway. The better rule is social fit: a stranger is normal on a public hallway and out of place in a private bedroom.

A person fits public norms and breaks private expectations. ✓ — Right. An empty public hallway can feel uncertain because no social or environmental cues confirm that everything is normal. One other person supplies that cue: in a public hallway, a stranger is expected, so the scene becomes more predictable. In a private bedroom, the same person breaks the expected social pattern, so the presence itself becomes the alarm.

A person provides a stable visual reference for room size. — Plausible, because a person can act as a visual reference for judging distance or room size. But size perception does not explain why the same person reverses from reassuring to unsettling. The reversal depends on whether the setting makes that person socially expected, not on the ruler-like cue they provide.