Why do dogs suddenly bark at nothing?
Dogs can hear and smell things we can’t (as easily and rapidly) sometimes. But that doesn’t guarantee that they’ll exhibit any type of response.
Barking, however, is a normal and common reaction that dogs will practice for various reasons. Most commonly to alert others around and/or to let whatever the trigger was know that the dog is aware of you and it would probably be best to go away.
Growing up I would always hear “Perro queue ladra no muerde.” Which in Spanish means “A dog that barks doesn’t bite.” and that’s because a dog that barks is communicating to you clearly that giving space and staying away is likely the best way you can proceed and avoid any confrontations. I find most bites tend to happen with the quiet, unsure/timid dogs that have used other, not as clear means of communication to let you know that you shouldn’t approach, and those being ignored left no other option but to bite t convey a clear message.
And so it’s important to note that a dog barking is one that is asking others to stay away or to alert you. But in either case, the best thing you can do is thank your dog for clearly communicating to you and helping them walk away to help them regulate their own emotions, which will help end the barking.