The butler escorted him to the library, and entered after him. Musard was in the room alone, standing by the fireplace, smoking a cigar. He looked up as Caldew entered. "I have just learnt something which I think you ought to know," he said. "The information comes from Tufnell. He tells me that while he was going around the house this afternoon he found the outside door of the back staircase unlocked." "Do you mean the door at the bottom of the staircase in the left wing?" asked Caldew. "Precisely." "I understood from Miss Heredith that this door was always kept locked." "So it is, as a rule. It was only by chance that the butler discovered this evening that it had been unlocked. You had better explain to the detective, Tufnell, how you came to find it unfastened." "I was going round by the back of the house this evening," said the butler, coming forward. "As I passed the door I tried the handle. To my surprise it yielded. I opened the door, and found that the key was in the keyhole, on the other side. I locked the door, and took the key away." "What time was this?" inquired Caldew. "A little before six--perhaps a quarter of an hour." "Is it your custom to try this door every night?" "Oh, no, it is not necessary. The door is always kept locked, and the key hangs with a bunch of other unused keys in a small room near the housekeeper's apartments, where a number of odds and ends are kept." "When was the last time you tried the door?" The butler considered for a moment. "I cannot rightly say," he said at length. "The door is never used, and I rarely think of it." "Then, for all you know to the contrary, the key may have been in the door for days, or weeks past." "Why, yes, it is possible, now that you come to mention it," said the butler, with an air of surprise, as though he had not previously considered such a contingency. "The key had been taken off the bunch?" "Yes." "Do the servants know where the key is kept?" "Some of the maidservants do. The back staircase is occasionally opened for ventilation and dusting, and the maid who does this work gets the key from the housekeeper." "Who has charge of the room where the keys are kept?" "Nobody in particular. It is really a sort of a lumber-room. The housekeeper has charge of the keys." "Thank you; that is all I wish to know." The butler left the room, and Caldew looked up, to encounter Musard's eyes regarding him. "Do you think this has anything to do with the murder?" Musard asked. Caldew hesitated for a moment. It was on the tip of his tongue to reply that he attached no importance to the butler's statement, but professional habits of caution checked his natural impulsiveness. "I want to know more about the circumstances before advancing an opinion," he replied. "Tufnell's story was rather vague." "In what respect?" "In regard to time. The door may have been left unlocked for days."