Ghosts?

My wife's dog is a small dog and like most small dogs likes to bark at things. People that walk by the house are especially good for barking at. I guess any excuse is a good excuse, more so when the neighbors dogs that are outside are also barking. Fun to see which dog will get the last word!
However there are times when this little dog barks at nothing. He stares intently as if he is seeing something but when I look there is nothing there. No movement from an open window breeze or anything, just nothing there.
I have to wonder if he is seeing the spirits that have followed me from my old house. There were several there at that address and they were, for the most part, friendly and somewhat protective of me. I know that at least one of them did follow me when I moved as I occasionally see the shadow as it crosses my bedroom. So, I have to wonder if this is what the dog is barking at when I see nothing there, just some spirit.
The bad part is that the dog sometimes greatly disturbs my daughter with his barking and she wonders what he is barking at. I have not told her that there are spirits in this house because she was very much disturbed by the ones in the old house. She thinks the spirits are out to get her.
Whatever the dog senses is not frightening to him or he wouldn't bark and whimper the way he does. He would run and hide as he is a smart little dog. I just wish he would bark a couple of times and stop. He however would rather bark quite a few times in several places before he stops.
So, I have to wonder, does the dog see the spirits that humans cannot see?