Recently, my youngest son and I watched the movie, Up (great animated movie, by the way). In what part, this dog with a voice maker (turns barks into human language, conveniently English) would frequently stop everything, become totally still, look in the distance, and say, "Squirrel". After a few second, normal behavior would resume.
My dogs do something similar.
Out of the blue, they will spot something and start to bark. All attention and body language is focused on the barking target. 95% of the time, I have no idea what they are barking at. But are they insistent. Is it a squirre, dog, other animall? Do they hear something beyond my human hearing? Did a leaf fall by the window? Or are they simply crazy?
I generally think it's that they heard something, but I'll never know. Unless I can find a voice maker that interprets dog barks into English!