Plato and Aristotle discuss techniques for observing people without being detected, referencing alleged CIA training.

A meme featuring a detail from Raphael's School of Athens, showing Plato and Aristotle with speech bubbles discussing how to look at people so they feel watched or don't notice they're being watched.

The CIA allegedly trains their people not to look at the back of someones neck when they're following them, since people can sense when they're being watched. You can try it for yourself, a few times I've looked intently at the back of a friend's head in a crowded area, while they're facing the other way. If they are sensitive they will turn around and look at you, subconsciously realizing you were watching them. You can take things one step further; look at someone for 1-2 seconds, avert eyes for 1-2 seconds, then look again for 1-2 seconds, repeat. Works through a window, which makes it hard for the observee to understand they are being seen, allowing them to act natural instead of trying to see who's looking at them. If you do this on a sensitive person, they will dance like a puppet, changing their stance and shifting their bodyweight every time you look at them or away.