This is the schtick to not be surprised.
Even when you seem to be paying complete attention to something else, your senses are working overtime looking out for trouble. Hidden dangers such as enemy ambushes, falling rocks, attacking poisonous snakes and ticking bombs hidden underneath cars rarely surprise you. When the GM makes a check on your behalf to see if you spot such things, you get to use your Guns instead of your Perception.
This bonus does not apply to finding a hidden thing or opponent, only to avoid being surprised in dangerous situations.