They know something isn't right but they are trying to get confirmation through another one of their senses.They didn't likely have a snoot full of human scent or they wouldn't have hung around.Their curiosity of course often times gets them killed.Most often if they see something that is not visually to their liking they will circle down wind of the thing(you or your blind)trying to confirm with their nose what their eye's are suspicious of.This typically only happens with younger deer or does(I said typically!)Big bucks are not awfully curious when they sense something is not right.