Unexplained knocking at the front door. 9/24/11.

Tonight, right after I had gotten home from Adele’s there was a knock on the living room door.  Since I was in the living room, I immediately opened the door.  No one was there.  Zoey had started barking as soon as we heard the knock.  She ran outside barking.  She couldn’t find anyone either.  No sooner had I closed the door, the knock sounded again.  I immediately opened the door and Zoey again ran outside barking.  The neighbors were sitting on their front doorstep and I hollered at them asking if they had noticed anyone in the yard.  They didn’t.  Was this Jeff or Ron?