system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do people sense that a situation or person feels "off"?
gpt
There's two levels in a person's perception. There's the stuff you outright notice and the stuff that you don't quite consciously realize. Examples of the latter for situations might include a slight breeze that your skin feels but that you don't really notice or a curtain that's slightly out of kilter compared to the last time you were in a room and it makes a different pattern that indicates someone might have been in there. Examples with respect to people might be someone who changes the nature of their eye contact or someone who's a little twitchy and you're not aware of a reason why they should be, or when someone gives you direction to do something and it just doesn't seem to make immediate logical sense.

Whether you directly notice it and consciously evaluate it, or it's a collection of not-quite-directly-observed little things that add up to a perception that makes you a little antsy, those types of things can make you get the feeling that something's "off".