When you're scared, your body physiologically changes, your body chemistry changes, you emit phermones. Some people visibly sweat. There's a difference between stress sweat and heat/body temperature sweat. Stress/fear/anxiety sweat is like skunk spray, it's that nasty/stinky funky ass sweat. Physical sweat don't have that smell, you'll just smell like dirt/outside/cologne/body spray/clean laundry/deodorant or whatever and if you're already funky then you're going to smell funky.
People dont understand how strong and sensitive dogs noses are. They smell you before you can even acknowledge then. They smell you before you even know what's happening in real time.
If you ain't scared, you ain't scared. And they know. I'm not #dogset but I give props where it's due.