Hello? Who's There?

Well, the footsteps have started again! Yesterday I was sitting at my computer and I thought someone hade walked in. Josh and Susan are out of town, and I can hear Greg on his crutches coming up the steps a mile away. So I knew it couldn't be them. But the steps sounded like the were coming from the door right behind me, walking toward me. And of course, when I turned around no one was there.