Shadow people?

Today, at my wife's store, a strange happening. We hadn't been open very long; maybe an hour. My wife was working at the checkout counter and I was working on a clock in the backroom area. My wife came into the work area and said the strangest thing had just happened. As she was working on some papers at the counter she saw someone come in the door right beside the check-out. She gave her typical salutation and offered assistance if such were needed. When she looked up, no one was there. I heard her salutation where I was working and naturally assumed some one had come into the store. At this particular time there were no customers in the store. There is no way shadow or person could have crossed that threshold. She said that whatever it was passed through from the door well into the store. No shadows would be possible from that door. A person would not have been able to pass out of sight that quickly. The front display windows are well away from the check-out counter and are so well stocked that outside shadows or reflection would have been nonexistent in that area of the store. I said, "I heard you greet somebody, but I didn't hear the door bell ring (A rather loud brass bell sounds when someone comes in or leaves). "I saw someone or the shadow of someone come in so I greeted them. But nobody was there", my wife replied. "Then come to think of it, I didn't hear the bell either". We get weird things in this old building from time to time. I guess this was just another one.