There's actually a psychological name for that -- "viewfinder invulnerability". Photographers focus on getting their shot to the point of forgetting about the real world. The classic example is a photo taken on a World War II carrier deck by a photographer assigned to the ship. It's a spectacular in-flight picture of a Japanese bomb that's just about to hit.

Yes, they found the image in the guy's camera afterward. He wasn't around to develop it.