Just offering an anecdote, but my parents' dog, a 50-lb lab mix, has never received training but is somehow able to tell whether someone on the porch is a stranger or not. Even when I come to visit after months of being away, she won't bark, but she'll bark at strangers as soon as she hears them on the porch or when they open the screen door. The windows on the front of the house are always covered (no silhouettes), and she doesn't have line of sight to the front door where the blinds are usually down. I'm guessing it's her excellent sense of smell or we have some kind of sound pattern that she can recognize (nothing obvious like keys jingling).

So I'm sure some dogs could be trained that way if desired. At a minimum they could be trained to bark only during the workday when no one's home or at night when legitimate visitors will be rare.