Why was Major Metcalf in the cellar when Mrs. Ralston screamed?

Another brutal murder has taken place at 24 Culver Street; Mrs. Boyle has been strangled. Already on the scene due to investigate the murder of Maureen Lyon, Sergeant Trotter questions all of the guests to get to the bottom of things. Everyone is a suspect, and everyone seems to have something to hide. Major Metcalf seems somewhat particularly evasive and suspicious when he's being questioned by Trotter. When Mrs. Boyle was strangled, Metcalf was down in the cellar—or so he claims, at any rate. However, his explanation is more than a little lacking in credibility. He claims, somewhat bizarrely, that he was exploring the cellar because he was interested in the house's architecture. But Trotter is not convinced; the Major, like everyone else, is under suspicion.