‘All right, I’m supposing it. So what?’

The Sergeant caressed his chin. ‘I haven’t worked it all out, but it does strike me that he may have thought he’d got to leave that door locked when he left the room.’

‘Why?’

‘Might be the time element, mightn’t it? He may have thought that if anyone was to come along and try the door a minute or two later, and find it unlocked, he’d be whittling down the time of the murder a bit dangerously. I don’t say I quite see –’

‘No, nor anyone else,’ interrupted Hemingway.

‘There might have been a reason,’ persisted the Sergeant doggedly.

‘There might have been half a dozen reasons, but what you seem to forget is that it isn’t all that easy to turn keys from the wrong side of the door. If the door was locked from the outside, the man who did it must have provided himself with a tool for the purpose. He couldn’t have done it extempore, so to speak.’

‘He could, by slipping a pencil through the handle of the key, with a bit of string attached.’

‘He could, but we haven’t any evidence to show that he did. In fact, we’ve plenty of evidence to show that he didn’t.’

‘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.’

‘The more I think of it, the more that ventilator looks to me like a snare and a delusion,’ said Hemingway. ‘It’s a good seven foot above the floor, to start with, and too small to allow an average-sized man to squeeze through it, to go on with.’