Dog barking at nothing
A dog barking at nothing was said to sense the unseen as far back as Norse myth. In fact it hears and smells a world far larger than yours, and the moment says something about the limits of what you yourself can perceive.
What it may mean
Norse myth said that dogs could see the goddess of death when she was invisible to people. The everyday truth is less eerie and just as interesting: for your dog, the empty corner is not empty. Dogs hear sounds far higher and fainter than people can and smell traces that are hours old, so mice in the wall, a fox two backyards over or the whine of a charger can be as real to them as a knock on the door is to you. The barking only looks pointless from inside human senses. That is worth remembering with people, too: when someone reacts to "nothing," they may be responding to something you simply cannot perceive, a tone in a voice, a memory, a pressure you do not feel. Before deciding a reaction is unreasonable, it can help to ask what they might be sensing.
Depending on how you saw it
Barking at a blank wall
A dog that barks or scratches at one spot on the wall is often hearing mice, insects or water moving inside it. The wall is quiet only to you. Is there an objection you waved off because you could not hear what the other person heard?
Staring at an empty corner
Dogs sometimes fix on a corner and freeze, ears forward, tracking a sound or a draft of scent. People have filled that stare with ghosts for centuries. What blank spaces in your life have you filled with stories that may not be true?
Barking at a sound you cannot hear
Dogs can hear another dog barking blocks away, or a high whine from electronics that people do not notice. They are answering a conversation you are not part of. Which conversation around you are you only half hearing, and does it matter?
Barking out the window at night
At night a dog may bark at the window over a raccoon, a deer or a cat crossing the yard in the dark. It sees movement you miss and smells what passed an hour ago. Who keeps watch for you in ways you rarely notice?
Why it happens in nature
Dogs hear sounds well above the range of human hearing and their sense of smell is many times keener than ours. A dog barking at nothing is often reacting to mice inside a wall, a distant dog, a gust carrying a scent or the whine of an electronic device. Bored or anxious dogs also bark more, and some bark simply to be noticed.
What folk beliefs say
Scandinavia (Norse myth)
In Scandinavian mythology, Hela, the goddess of death, showed herself to dogs when she was invisible to human eyes, and that was said to be why dogs howl.
Source: Cora Linn Daniels; C. M. Stevans (eds.), Encyclopaedia of Superstitions, Folklore, and the Occult Sciences of the World, vol. 2 (1903), vol. 2, p. 613. View the page
To reflect on
Whose reaction did you call unreasonable before asking what they might be sensing?