system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can we "sense" someone's presence in a room without seeing or hearing them?
gpt
We can't.

However, you can "hear them" even when they're not making any noise.  Simply having a large mass move into close proximity to you will change the acoustic properties of the room, which can slightly change the volume or timbre of ambient sounds.  We tend to pick up on this on almost a subconscious level.

Also, vague moving shadows, subtle reflections off various surfaces, etc. can give a visual indication even when you're not looking at the person directly.