The reason: a bump in the night. The early night, to be sure, but it was dark and it was rainy and the bump sounded like something big had fallen upstairs.
Was it a cat burglar come to get my jewels (an errand sure to disappoint, I might add)? We crept upstairs to check it out, entered each room carefully, and there was — nada. No box had fallen from a shelf, not a thing out of place.
There's a chance this was an outside noise mimicking an inside one. But I doubt that. I'm going to assume it was just a friendly poltergeist messing with us a little, taking advantage of this old house, with its creaks and groans, sending us a message — that we are not alone.