Dogs hone in on any movement even a blade of grass in the breeze gets their undivided attention as they know something, which could be an animal, is making the grass move! That is why animals such as deer will often stay frozen in place as we pass near them to avoid provoking a chase which definitely is effective as a lot of times the dogs don’t even notice them when they remain still, but deer like some other wildlife will watch us like a hawk in a ready stance if they sense the dogs are going to chase they instantly takeoff.