Dodeedodee dodeedodee (Twilight Zone Theme)
It's kind of like when you're working in the woods and see the deer around you all the time. They ignore you even when you stop to watch them, sometimes they're curious to see what the sawing and chopping is all about, but they seem to know that you are no threat to them. Put a rifle in the truck, though, and you see nothing but tail flags when you take it out and load it. My personal theory is that when you are actually looking to shoot one, the chemical scents that your body produces change and their noses pick up the difference. That might explain the fox, too, assuming the wind was in his/her direction. They certainly have good enough noses to detect the difference, better than deer do, I think. Either that or they can flat out read your mind, still within the realm of possible as I don't think it can be disproved, but a little too far into the metaphysical realm for this yankee.