(photo by expertinfantry)
Have you ever been so hungry that you can smell food even when you’re a room away?
Your nose picks up on the passive signal that is the heavenly scent your favourite lasagna is wafting from the kitchen, and you promptly get off your lazy bum to go chase after it. Even when you don’t see the physical food just yet, but you can pinpoint where it is just by following the trace.
That’s technically how seeker missiles work.