superfox is actully got it on the nose. the only thing that superfox may not be thinking about? still the only thing I would add to superfoxs answer is they may be reading from some person in danger a block or two down the rd. be it criminal,house getting broke into, neibor may have fallin and he's kinda old and he's hollering for help,but you can't hear him. the dog can tho. same with sirens and airplanes to helicopters. you truely dont know exactly what it is. but the dog does. so a good thing to do when a dog is barking for no reason, that's as you were saying, at nothing? but when they are barking? take a second and come up to the dogs side and ask him. what's up buddy? what is it? then just stand and listen? who knows it could be nothing but still I would say good boy to the dog. keeps the dog on the watch still. but it could be a day you save someones life. from the barking of your dog. or better yet an intruder?
remember a dog can make a mind to do many things. the tone of the bark will best answer . territorial,scent, normally its a sound off to let others know this is his turf. hearing is above and below. give him some possesions and walk him outside and along his outer perimeter. he might enjoy making u wonder
When my silly dogs bark, there's almost always a noise they hear, but I have to stop and really listen to hear. It's there, just not noticeable to our thick ears. Try shutting them in the garage and turning on a radio so they won't hear the far away sounds.
Because humans cannot see or hear on the level that dog can, we cannot detect or pick up on somethings that they can. To you the dog may simply be barking at nothing but remember that they see better than us and that they can hear higher pitches that us. So when your dog barks they is usually a reason, just not a reason that we know and comprehend.