"Were there any finger-prints on the key?" asked the Sergeant.
"Old Herriard's, and the valet's, considerably blurred. Just what you'd expect."
The Sergeant sighed. "Nothing seems to lead anywhere, does it, sir? I'm blessed if I know how to catch hold of this case."
"We'll go back to the station," decided Hemingway. "I'm going to have another look at that key."
The key, however, revealed no new clue. It was a large key, and it had been lately smeared with vaseline. "Which makes it still more unlikely that it could have been turned from outside," said Hemingway. "To start with, I doubt if any oustiti would have gripped such a greasy surface; and to go on with, we'd be bound to see the imprint of the grooving on the grease. It's disheartening, that's what it is." He scrutinised the handle through a magnifying glass, and shook his head. "Nothing doing. I'd say it hasn't been tampered with in any way."
"Which means," said the Sergeant weightily, "that whoever locked that door did it from the inside."
"And then dematerialised himself like the spooks you read about. Talk sense!"
"What was to stop him hiding in the room until the body had been found, and then slipping out unnoticed, sir?"
"Nothing at all. In fact, you might have got something there, except for one circumstance. All the members of the household were accounted for at the time of the discovery. Think again!"
"I can't," said the Sergeant frankly. "Seems as though we've got to come back to the ventilator."