Depending on proximity, you can detect air pressure changes in your ears and on your skin, subtle smells, lighting change queues, and soft sounds to trigger that feeling. Some folks have called it the ‘animal eye’, a combination of your senses to produce an virtual additional ‘sense’ that your brain uses to inform you of things you can’t easily and consciously detect otherwise. It is what makes the hair raise and you get goose pimples sometimes if a shadow crosses you from something above.
Of course, sometimes it doesn’t work – you’ve probably been snuck up on before. Private Investigators are a thing and watch people through binos without them noticing. Sometimes it triggers when there’s nothing there and you get the ‘heebie jeebies’ feelings.
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