At once I thought of Denson's disappearance unobserved by the housekeeper. Could this be the key of some private exit from the office building? I resolved to test that conjecture first, and it turned out to be the right one. Being successful so far, of course I turned to the other new key and tried that, as you saw."
"But what of that triangular mark on the man's forehead?"
Martin Hewitt became deeply thoughtful. "That," he said, "is a matter wholly beyond me at present, as indeed is the whole business of the murder. Whether we shall ever know more I can't guess, but the matter is deep—deep and difficult and dark. As to the mark itself, that seems to have been impressed from an engraved stamp of some sort. It is a plain equilateral triangle in red outline, measuring about an inch on each side. It is in a greasy, sticky sort of red ink, which may be smeared, but is very difficult, if not impossible, to rub away. What it means I can't at present conjecture. I have told you my reasons for not thinking it the sign of any gang of criminals. But whose sign is it? Surely not that of some self-constituted punisher of crime? For such a person, with no risk to himself, could have handed Denson over to the police, if he knew of his offence. Can he have been murdered by an
