Why does Saul’s grandmother claim that they’re close to Saul’s great grandfather’s trail, though Saul himself can’t see it?

Saul needs to use more than sight to see the trail. Saul closes his eyes and takes a deep breath of air. He can hear the faint "hiss" of the river. When he opens his eyes, he can see the faint outline of a trail in the snow. Saul needed to use more of his senses than eyesight.