Eric isn't a dog that barks a lot, only really when someone comes to the door or if I ask him if he wants to choose a bedtime biscuit 
Postman and deliveries come here at any time and obviously not every day but Eric starts growling before they can have set foot on the path (no gate to give him a clue either) as it' three strides from pavement to front door. We have blinds at the front so he can't see that they are coming and he never barks when they are on the street but not delivering to us so how does he know?
Are your dogs the same?