Every have your dog wonder where the barking is coming from? Sometimes while watching TV a dog starts barking, it sounds as if it is the room. The dog's response is to start barking.
Tanner learned, when we lived in a home with a doorbell, that every time the door bell rang there was someone at the door. Our house now doesn't have a doorbell, but every time a door bell rings on TV he starts barking and running for the door. It's amazing how they can train themselves with positive reinforcing situations.