With their hearing or vision if they aren't sure about something they will attempt to use another sense to confirm the danger,almost always they are looking to confirm with their nose whether or not they need to flee.When they get a snoot full of you they are out of there,no need for confirmation.It is their sense of smell,the others not even being close.
It would probably depress the heck out of all of us to know the number of big bucks that have slinked away with our odor intheir nostrils,with us never having seen a glimpse of them.