Strange "Illusion"

 More than one strange item today. After my wife and I closed up the store we headed home. We both drive and I got home a few minutes before my wife. I unlocked the rear, western, kitchen door and was greeted by my dogs. I walked through the kitchen into the short downstairs hallway where the backstairs go up to the second floor. As I stood in the hall, taking off my coat and hanging  it up I looked into the Dining room to the door that leads into the parlour. My house, like most old houses, has large interior doors that are just short of one half inch off the floor. I'm  sure that if you have ever seen doors that do not hug the floor or carpet you  realise that anything beyond that door moving close to the door is evident as moving shadows on the other side of the door, As I watched, I saw definite movement in shadows under that door. This gives you a real start. Moving forward quickly, I opened the door. Much to my relief nothing was there. But what made the movement and shadows I saw under the door?  It was possibily an illusion; but it can still give you a start and what created the illusion to begin with.