This is a very interesting topic.
Personally, I feel animals look at people as just another large predator. They run from people because of their size! Not from a fear of human scent. Good grief, if it was a human scent thing, animals would be running all the time. Yet, fox will come into a farm yard at night, and so forth.
I think the whole human scent thing was something cooked up back in the late Nineteenth Century by trappers who could catch fox reguarlly. They were hounded by wanna be fox trappers, or thought they were. As a way of throwing those wanna be trappers off the trail, human scent was bandied about as a real issue.
This was printed so many times, in the available books and magazines, it became "fact".
Then the 70's came and the supply dealers started the notion you had to use fox urine to hide human scent. This was nothing more than marketing ploy. Create a need and fill it.
Plus, Paul Dobbins cornered the fox urine market and wrote all the fox books that came out in the 70's and 80's. This explains why they all seemed to read the same way.
As long as the person is not there when the animal is, then human scent is just so much back ground noise-so to speak.
Think of it this way, you hear a train whistle while standing on your back porch, you will pay little attention to it. Why? Because it is of little threat to you. But, your truck stall on a rail road crossing and you hear a train whistle, you will take a very keen interest in it!
Also, the emotional state a person is in will change the scent they release. If they are upset, sick or whatever, the fox pick it up and avoid the place.
My Uncle Earle was 10 years older than dad, he taught dad how to trap. But, he never was able to catch fox. Dad, on the other hand, had no trouble catching fox.
I remember back in the late 50's, dad would show Uncle Earle how to make a fox set, and he gave Uncle Earle some of the fox lure he used. Uncle Earle never caught a fox.
Uncle Earle was a big man, but he let everything bother him-he was all the time peeved about something. Dad on the otherhand, was slow to get mad and did not get upset so easily.
I think the fox picked up on that and were less afraid of a set dad made than one Uncle Earle made. Why would fox react this way? Self preservation.
Think about it for a moment, you are in a large city. A person you do not know lean against a lamp post and heave his guts up. What do you do? You avoid him! Why? Self preservation, you do not want to catch whatever he has that made him sick. Animals are the same way.
Add to this, the lure makers can not help but get some of their own scent into the lure. The only way they could keep it out is to have a "clean room" like computer chips are made in. This would cost so much, lure would sell for $10,000.00 per ounce, not much lure would be sold.
A good reason why a great lure, made by one individual becomes a real dog when that maker dies and the formulae are sold to the highest bidder.
I bet the lure maker's body odor is a major ingredient in his lure. Plus it blends in with the trapper's odor to attract the animals. Or cause the animals to shy away.