I seem to remember reading about a study that had looked into that - I'm thinking it was for a class, but it may have just been internet reading.
If I'm remembering correctly they found that "tunnel vision" is - at least in part - selective memory after the fact.
When you're stressed out you're likely going to be focused on one thing. Being so focused will tend to make you less observant at the time, but has an even greater effect on your ability to remember the things you weren't focused on later. It's a lot harder to override our senses than our memory.
The same thing happens any time you're extremely focused on something. Driving in the snow you (hopefully) still notice that there are other cars on the road and drive accordingly, but after you get home you'd probably be hard pressed to remember any of them specifically.
Those who would give up Essential Liberty to purchase a little Temporary Safety, deserve neither Liberty nor Safety. ~B. Franklin