One of the things I first learned when I read Tom Brown Jr’s The Tracker, was the concept of splatter vision.This technique was first used by Native Americans long ago to track game. This is basically the same tactic that animals themselves use to spot movement and/or danger.
Basically, this is a technique where you let your vision “spread out”. You look toward the horizon while expanding your peripheral vision. The trick is to not focus on any one thing in particular. If you are doing it right, your vision will seem a little fuzzy and since your eyes are not focused on any one thing, they will be more sensitive to movement.
Princeton University’s Outdoor Action has a very good description of this tactic and a description of the difference of how humans and animals view their surroundings:
We tend to use focal vision about 95% of the time and wide-angle vision only 5%. Animals use the reverse (5% and 95%). To use wide-angle vision you want to take in all the information from your peripheral vision constantly then focus down when needed. Concentrate on the entire picture, mentally blocking out information to focus down.
The primary thing that gives you away (or an animal) is movement. Focused vision doesn’t pick up movement whereas wide-angle vision makes the eye reactive to movement. When you notice movement then focus down to that object. And once focused, keep tracking that animal visually very closely so that you don’t lose it. Keep this process in mind! This is how the animals look for you. Anything that is out of the natural order, movement, shadow, or noise attracts their attention and they focus on it.
At night using wide-angle vision utilizes all the peripheral areas of the eye which are more sensitive to low levels of light. This improves night travel and seeing animals. It will allow you to notice nighttime animal movement. Flashlights cause focal vision which restrict your sensitivity to movement. At night a wind will blow things in one rhythm. Anything moving contrary to that rhythm, check it out with focal vision. — Guide to Nature Observation & Stalking (PDF)
This is the same tactic taught to FBI agents to spot threats in a large crowd of people. This tactic involves scanning a crowd by looking into the distance and not focusing on anyone in particular.
Once the agent fixes a general gaze on the crowd, he or she looks for any deviation or change. By balancing directed and undirected scanning, a single agent can spot signs of trouble across a fairly large area. — (source)
To practice this technique, look forward without focusing and let your vision widen out. Try to notice things in your periphery while not focusing on them.
Pay attention to nothing and you will see everything.