Mine used to bark alot too, and I'm not sure if I ever discovered what she was barking at...but she finally grew out of it. 

buuuut......

I don't want to call the police about the situation, but if its another barking-fest at 2:00am, I might have to. Also beginning to wonder if I'm the only one complaining about it..... but I think, they are leaving home, and leaving the dog out while they are gone (which is all the time)..................

ohh the frustration!