When the snow falls.

While out of your line of sight, you hear the screen door open and close quickly. By the time you get to the door, it is closed. You swing the door open as fast as you can. Then, you open the front door. While you stand there, in the cold and blustery wind on winter, you don't see anything but the yard and an empty driveway. Nor were there any footprints in the newly fallen snow.