“Where?
“Outside the billiard-room door.”
“Outside? But it was inside when we looked just now.”
“Exactly.”
“Who put it there?”
“Obviously Cayley.”
“But—”
“Let’s go back to this afternoon. I don’t remember noticing the billiard-room key at the time; I must have done so without knowing. Probably when I saw Cayley banging at the door I may have wondered subconsciously whether the key of the room next to it would fit. Something like that, I daresay. Well, when I was sitting out by myself on that seat just before you came along, I went over the whole scene in my mind, and I suddenly saw the billiard-room key there—outside. And I began to wonder if the office-key had been outside too. When Cayley came up, I told you my idea and you were both interested. But Cayley was just a shade too interested. I daresay you didn’t notice it, but he was.”
“By Jove!”
“Well, of course that proved nothing; and the key business didn’t really prove anything, because whatever side of the door the other keys were, Mark might have locked his own private room from the inside sometimes. But I piled it on, and pretended that it was enormously important, and quite altered the case altogether, and having got Cayley thoroughly anxious about it, I told him that we should be well out of the way for the next hour or so, and that he would be alone in the house to do what he liked about it. And, as I expected, he couldn’t resist it. He altered the keys and gave himself away entirely.”