WARNING: If you get scared easily, DO NOT READ THIS POST

Once I was home alone when there was what looked like a camera flash out my glass back door. Thinking it was one of the neighbors, I opened the door. Then there was a sound of knocking on hollow wood at my right, where the door shut; it was a sliding door. Getting a little scared, I called some of my friends. Everyone I had called so far said it was a camera flash and the a woodpecker or other logical explinations. But I wasn't sure. Then I called my closest friend and when she picked up the phone, she sounded surprised. "I was just about to call you!" She said. "I just saw a flash at my door, and then a knocking noise! I'm sure you're going to throw some logical e-" I told her what had happened to me. We to this day do not know what happened.