“ ‘Because during the last few nights I have always, about three in the morning, heard a low, clear whistle. I am a light sleeper, and it has awakened me. I cannot tell where it came from⁠—perhaps from the next room, perhaps from the lawn. I thought that I would just ask you whether you had heard it.’

“ ‘No, I have not. It must be those wretched gipsies in the plantation.’

“ ‘Very likely. And yet if it were on the lawn, I wonder that you did not hear it also.’

“ ‘Ah, but I sleep more heavily than you.’

“ ‘Well, it is of no great consequence, at any rate.’ She smiled back at me, closed my door, and a few moments later I heard her key turn in the lock.”

“Indeed,” said Holmes. “Was it your custom always to lock yourselves in at night?”

“Always.”