Behind the Bronze Door by William Le Queux

Alix Stothert, the mysterious manager of the Metropolitan Secret Agency, lived in the house with the bronze face, on the top floor, and so did his partner, the woman known as Madame Camille Lenoir.

They were a sinister couple, of whom nobody seemed to know anything, and the police, when questioned concerning them, as they had been on more than one occasion, refused to make any statement. That the police looked upon them with no favorable eye was generally admitted by those in a position to know, and the inference naturally drawn was that Scotland Yard was jealous of the Metropolitan Secret Agency's extraordinary success in making discoveries which led to the arrest of criminals while the police failed to obtain even clues.