The adventure of the speckled band
As my sister was leaving me, she paused at the door and looked back.
"Never," I said. "Why do you ask?"
She replied, "Well, for the last three nights, always at about three in the morning, I have heard a low clear whistle. I am a light sleeper and it has awakened me. I cannot tell where it has come from - perhaps from the next room, perhaps from the lawn. I thought I would just ask if you had heard it. You don't whistle when you sleep, do you?"