"You didn't happen to lose your latch-key during your midnight stroll, sir?"
"No, it was only when I put my key in the door that I discovered it to be open. I have a spare latch-key which I keep for emergencies, but when I went to look for it just now the key was not to be found. When I came back the house was perfectly quiet."
"What family have you, sir? And what kind of servants?"
"There is only myself and my mother, with three maids. You may dismiss any suspicion of the servants from your mind at once. My mother trained them all in the old vicarage where I was born, and not one of the trio has been with us less than twelve years."
"That simplifies matters somewhat," Marley said, thoughtfully. "Apparently your latch-key was stolen by somebody who has made careful study of your habits. Do you generally go for late walks after your household has gone to bed, sir?"
David replied somewhat grudgingly that he had never done such a thing before. He would like to have concealed the fact, but it was bound to come out sooner or later. He had strolled along the front and round Brunswick Square. Marley shrugged his shoulders.
"Well, it's a bit of a puzzle to me," he admitted. "You go out for a midnight walk—a thing you have never done before—and when you come back you find somebody has got into your house by means of a stolen latch-key and murdered somebody else in your conservatory. According to that, two people must have entered the house."
"That's logic," David admitted. "There can be no murder without the slain and the slayer. My impression is that somebody who knows the ways of the house watched me depart. Then he lured his victim in here under pretence that it was his own house—he had the purloined latch-key—and murdered him. Audacious, but a far safer way than doing it out of doors."
But Marley's imagination refused to go so far. The theory was plausible enough, he pointed out respectfully, if the assassin had been assured that these midnight rambles were a matter of custom. The point was a shrewd one, and Steel had to admit it. He almost wished now that he had suggested that he often took these midnight rambles. He regretted the fiction still more when Marley asked if he had had some appointment elsewhere to-night.
"No," David said, promptly, "I hadn't."