This is "ONE" of those very-very-very "tricky" threads for people to respond to. I am not accusing you of this "randyt" (because you and I go way back in forums) But this is exactly the type thread that would be/and often is created by employees of three letter agencies. So don't expect in depth super detailed responses.
I hang spawned-out propane tanks on my "Private" trails. They are pure gold for teaching what you ask. There was a time I would paint them different colors, for "friendlies" or "Bad'asp Uglies".
We live in a world where-in 99+ % are spending 101% of their lives "Intentionally Distracted" from reality. This is actually a "wonderful thing and a gift" for those who are "not".
I should offer a cheap course in this. I'll walk the students past spawning areas where I know we will have bear encounters, without first warning the student first. After they nearly get eaten, and pooped their britches, they will notice a flea farting two hundred yards away.
The short answer to your inquire, is spend abundant time in the "Danger Zone". As others have said, learn to observe, but "nothing" will encourage that like being in "danger". When you are a "Professional Hunter" you don't look for the quarry, you look for an "ear" tip that does not belong where it is. If all the leaves are still and one leaf is moving, you "notice" but don't lock on that leaf, you have trained yourself "not" to get tunnel vision. You have a "special" expanded visual width and depth, I can assure you that even after 40+ years of going into very thick vegetation to finish wounded Grizzly Bears, you will notice everything.
Sadly we are able to "generally" survive in a totally oblivious state. (Till we can't).
EDITED to add: At a point, you are no longer in your body looking and hearing, and smelling for danger, you are above your body (slightly) and you see, hear, smell, perceive things.......even things behind you, you are in a dimension few will ever experience. There is a "purity" and crispness that is intoxicating. (You "were" the hunter, now you are the hunted, and generally by an enity vastly more skilled).