“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.”

“But the library key was still outside. Why didn’t he alter that?”

“Because he’s a clever devil. For one thing, the Inspector had been in the library, and might possibly have noticed it already. And for another⁠—” Antony hesitated.

“What?” said Bill, after waiting for him to go on.