Quite a few years ago a study was conucted on the King Ranch in TX. Because they already had a large number of bucks radio-collared, the had the opportunity to approach the known location of these radio-collared bucks (walking directly at each buck's radio-signal). Through triangulation, they could watch for when each individual buck reacted to the person walking towards them.

When reviewing the data, they found exactly the opposite of what they had expected. The bucks that reacted first--when the person was farthest away--were the youngest bucks. Older bucks, and especially mature bucks, held tight. In fact, the most mature bucks held so tight to even the smallest patches of cover that the person walking toward them often had to literally kick the bucks up out of the cover (nearly having to step on the bucks). Interestingly, the person walking towards the buck knew about where the buck was due to his radio signal but often could not visually see the buck until he bolted from underfoot.

Now this study was done in TX brush country. Perhaps deer in other habitat would react differently. But I suspect this behavior is fairly universal. Mature bucks act like rabbits--hold motionless and let danger walk by--while younger, less experienced bucks run at first sign of danger.

A still deer, in even minimal cover, is extremely hard to see,
even if you know he is there. I watched a small 6 pointer bed
down about 40 yards from my stand a few years ago. It was in
fairly open woods, but everytime I looked back to check on him
I had a hard time finding him. Several time I would think he
left while I was watching in other directions, but I would
finally see a black nose or see a ear flick.

Totally amazing.

Something I sometimes do while on stand when deer come in and I know I'm not going to shoot any of them....I look away and then turn back around looking at the deer again. Often times, knowing exactly where the deer were before I looked away, I still have to search for them to see them again. They blend into their surroundings and when motionless are hard to pick out even then.