“I agree. But someone might have come in through the study window.”

“But that is just what you said did not happen?”

“I said that no one from outside could have come and left without leaving marks on the grass. But it could have been managed from inside the house. Someone could have gone out from his room by one of these windows, slipped along the terrace, in at the study window, and back again in here.”

Mr. Carlile objected:

“But Lord Mayfield and Sir George Carrington were on the terrace.”

“They were on the terrace, yes, but they were en promenade. Sir George Carrington’s eyes may be of the most reliable”—Poirot made a little bow—“but he does not keep them in the back of his head! The study window is at the extreme left of the terrace, the windows of this room come next, but the terrace continues to the right past one, two, three, perhaps four rooms?”

“Dining room, billiard room, morning room and library,” said Lord Mayfield.

“And you walked up and down the terrace, how many times?”

“At least five or six.”

“You see, it is easy enough, the thief has only to watch for the right moment!”

Carlile said slowly:

“You mean that when I was in the hall, talking to the French girl, the thief was waiting in the drawing room?”

“That is my suggestion. It is, of course, only a suggestion.”

“It doesn’t sound very probable to me,” said Lord Mayfield. “Too risky.”

The Air Marshal demurred.

“I don’t agree with you, Charles. It’s perfectly possible. Wonder I hadn’t the wits to think of it for myself.”

“So you see,” said Poirot, “why I believe that the plans are still in the house. The problem now is to find them!”

Sir George snorted.

“That’s simple enough. Search everybody.”

Lord Mayfield made a movement of dissent, but Poirot spoke before he could.

“No, no, it is not so simple as that. The person who took those plans will anticipate that a search will be made and will make quite sure that they are not found amongst his or her belongings. They will have been hidden in neutral ground.”

“Do you suggest that we’ve got to go playing hide and seek all over the bally house?”

Poirot smiled.

“No, no, we need not be so crude as that. We can arrive at the hiding place (or alternatively at the identity of the guilty person) by reflection. That will simplify matters. In the morning I would like an interview with every person in the house. It would, I think, be unwise to seek those interviews now.”

Lord Mayfield nodded.

“Cause too much comment,” he said, “if we dragged everybody out of their beds at three in the morning. In any case you’ll have to proceed with a good deal of camouflage, M. Poirot. This matter has got to be kept dark.”

Poirot waved an airy hand.