Are you wearing rubber boots? IMO they are smelling your footprints in the snow. Deer have no way of knowing what made the tracks in the snow but they can smell very well. I had 26 deer walk in single file behind a mature doe several years ago. She was very alert and when she came to my tracks in the snow, she put her head down and smelled the tracks and raised her head and looked right in my direction. I was downwind and sitting in a tree about 200 yards away but she knew which way I was walking. She quietly turned around and walked back the way she came with every single deer in the herd following her. Mature does are slick!