Hehe.....While I wouldn't jump right to the conclusion that you have a ghost
Your dog could have been hearing anything
a mouse as someone already has suggested or maybe something outside
But dogs are sensitive to energies if you believe in that sort of thing
We did live in a house that had a resident ghost many yrs ago
and things would bemoved you would hear voices at night
and the little dog that we had at the time
would get up and bark and growl when there was nothing there
but what made it scarey was that she would back up like something was coming toward her and then she would run away
