“My theory is that Mark discovered the passage about a year ago⁠—the time when he began to get keen on croquet. The passage came out into the floor of the shed, and probably it was Cayley’s idea to put a croquet-box over the trapdoor, so as to hide it more completely. You know, when once you’ve discovered a secret yourself, it always seems as if it must be so obvious to everybody else. I can imagine that Mark loved having this little secret all to himself⁠—and to Cayley, of course, but Cayley wouldn’t count⁠—and they must have had great fun fixing it up, and making it more difficult for other people to find out. Well then, when Miss Norris was going to dress-up, Cayley gave it away. Probably he told her that she could never get down to the bowling-green without being discovered, and then perhaps showed that he knew there was one way in which she could do it, and she wormed the secret out of him somehow.”

“But this was two or three days before Robert turned up.”