Scanners: Narcissists Scan Others Constantly

Social Signals.
You give them whether you mean to or not, and those who have Narcissism issues ~ especially N's who are 'predators', or who are consumed with envy, control, or resentment ~ LOOK for them, watching and listening much, much more for social signals than other people do, ~ looking for a way in, for leverage, or for a chink in the armor.

If you pull up to a gas station and a Narcissist pulls up to the other pump, they ARE looking at you, doing an automatic quick-scan, even if they're not looking for any particular reason. Are you a potential mark, or a potential threat? Can they get your attention? Can they get away with giving you "advice"? Do you look anxious, hostile, calm, kind, or naive? Do they approve of you, or disapprove of you? Do you look like you'll respond favorably if they approach you (or do you look like you'll punch them in the face?) Do they find you attractive, too thin, too heavy, too tall, too short, too well-dressed, not well-dressed enough, weird, intimidating, friendly looking... etc... etc...

A non-narcissist would be exhausted by the amount of scanning and assessing a narcissist does on a regular basis.

They'll even look at you in your car from their car while driving on the highway.