Deer spook for the oddest reasons. The buck I finally killed I have had in range 3 times this season before thwacking him. Each time, something would go wrong...wouldnt slow down, chasing a doe, and one time...for no reason, he did exactly as you describe....came in from opposite direction as I did, wind blowing back where I walked in from, steady wind at that...and he was walking with his nose to the ground and he glanced over at the base of my tree....and bolted. I had two stand backpack straps leaned up against the tree, and I feel VERY confident this is what that buck spooked at the sight of.

IMO, if a deer catches your outline in peripheral vision, they will look up at you out of instinct before bolting...too many things in the woods for them to bolt without getting a look at first (as far as vision goes)....scent wise, game over at the slightest hint in most cases.

Just in my experiences though, no where near Schultzys 24, I've only been chasing em for give or take 8 years now. Only semi seriously for 3-4 and very seriously for the past 2.