I agree with DogWalker about the little changes.
I think people don't trust their gut because it seems unfounded and "just" a feeling, not evidence. But actually, I think what is going on is your brain noticing patterns way before you're consciously aware of it.
Take mentionitis - we all know it's not uncommon for someone to start mentioning someone they fancy. And when it's really obvious, it's not a gut feeling as such.
But what about when it's more than normal, more than that person was mentioned before, or more than someone else is mentioned, but not at obvious levels? I I think your brain is still doing the counting, recording, the statistical analysis. And far from being a "gut" feeling, if you could write down the numbers your brain has, it would be a very valid statistical analysis of significance.
Gut instinct seems mystical, but I think it's mostly a science we don't understand yet.
A different point: often people say they had a gut feeling. Sometimes, if you push them, I think what they had was actual evidence but they weren't ready to accept the consequences of it for whatever reason. How often do we see threads on here where women are "unsure" but go on to list what is, objectively, wrong behaviour?