"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."

"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.

"It's only guesswork. But I fancy that Cayley was thoroughly upset about
the key business. He suddenly realized that he had been careless, and
he hadn't got time to think it all over. So he didn't want to commit
himself definitely to the statement that the key was either outside or
inside. He wanted to leave it vague. It was safest that way."

"I see," said Bill slowly.

But his mind was elsewhere. He was wondering suddenly about Cayley.
Cayley was just an ordinary man--like himself. Bill had had little jokes
with him sometimes; not that Cayley was much of a hand at joking.
Bill had helped him to sausages, played tennis with him, borrowed his
tobacco, lent him a putter.... and here was Antony saying that he was
what? Well, not an ordinary man, anyway. A man with a sec