This is it. They're hard-wired instincts, and our unconscious workings piece them together so quickly we're not necessarily even aware of undergoing that process.
A PP mentioned growing up in a traumatic environment (my commiserations, me too) having really sharpened those survival instincts for her. I had the opposite experience. Because far from my trauma fine-tuning those danger signals, until I was well into adulthood mine were non-existent. What was this 'gut instinct' people spoke of? I had no experience of it.
My therapist had a theory about this one, too. You can't live in a perpetual state of fight, flight, freeze or fawn for any long duration. It would exhaust you and you'd be wholly unable to cope. Also, those instincts would be of no earthly use to you if you were living in a state of heightened awareness all the time.
So those instincts repress themselves. They switch off. For a reasonably sensible person I got myself into some ridiculously risky situations as a teenager and young adult. I was a constant source of worry to my poor mum.
I was diagnosed with cPTSD in my late thirties and had EMDR therapy: a real game-changer that's given back to me a life I never fully realised I'd 'lost' in the first place. It altered the holistic picture of my adult interactions, stopped me being a people pleaser and a 'yes' person amonst other more fundamental things. There were always things about myself that never added up: why a reasonably skilled communicator could be such a poor judge of character is a thing I frequently questioned about myself.
The therapy switched those instincts right back on. Imagine never having a gut instinct at all, then suddenly discovering it when you're into your forties. It's been a real revelation, that's for certain. I know of no peer-reviewed research that could support a word I'm saying; I only know that as a trauma 'patient' this has been my first-hand experience. What the human mind is capable of doing is a constant well of amazement: we really are fascinating animals.