“Well, it’s just a question of where people happen to keep their keys.  You go up to your bedroom, and perhaps you like to lock your door in case anybody comes wandering in when you’ve only got one sock and a pair of braces on.  Well, that’s natural enough.  And if you look round the bedrooms of almost any house, you’ll find the keys all ready, so that you can lock yourself in at a moment’s notice.  But downstairs people don’t lock themselves in.  It’s really never done at all.  Bill, for instance, has never locked himself into the dining-room in order to be alone with the sherry.  On the other hand, all women, and particularly servants, have a horror of burglars.  And if a burglar gets in by the window, they like to limit his activities to that particular room.  So they keep the, keys on the outside of the doors, and lock the doors when they go to bed.”  He knocked the ashes out of his pipe, and added, “At least, my mother always used to.”

“You mean,” said Bill excitedly, “that the key was on the outside of the door when Mark went into the room?”

“Well, I was just wondering.”

“Have you noticed the other rooms the billiard-room, and library, and so on?” said Cayley.

“I’ve only just thought about it while I’ve been sitting out here.  You live here haven’t you ever noticed them?”

Cayley sat considering, with his head on one side.

“It seems rather absurd, you know, but I can’t say that I have.”  He turned to Bill.  “Have you?”

“Good Lord, no.  I should never worry about a thing like that.”

“I’m sure you wouldn’t,” laughed Antony.  “Well, we can have a look when we go in.  If the other keys are outside, then this one was probably outside too, and in that case well, it makes it more interesting.”

Cayley said nothing.  Bill chewed a piece of grass, and then said, “Does it make much difference?”

“It makes it more hard to understand what happened in there.  Take your accidental theory and see where you get to.  No instinctive turning of the key now, is there?  He’s got to open the door to get it, and opening the door means showing his head to anybody in the hall—­his cousin, for instance, whom he left there two minutes ago.  Is a man in Mark’s state of mind, frightened to death lest he should be found with the body, going to do anything so foolhardy as that?”

“He needn’t have been afraid of me,” said Cayley.

“Then why didn’t he call for you?  He knew you were about.  You could have advised him; Heaven knows he wanted advice.  But the whole theory of Mark’s escape is that he was afraid of you and of everybody else, and that he had no other idea but to get out of the room himself, and prevent you or the servants from coming into it.  If the key had been on the inside, he would probably have locked the door.  If it were on the outside, he almost certainly wouldn’t.”

“Yes, I expect you’re right,” said Bill thoughtfully.  “Unless he took the key in with him, and locked the door at once.”