Behind the Throne by William Le Queux

The man whose brilliant career had ended, longed to open the safe and to see whether certain papers it contained had really been disturbed. But even if he possessed the key which he had flung into the Arno on that memorable night, it could not be opened on account of the piece of wire which prevented the lever from working. The truth was therefore withheld from him.

Sometimes he regarded Mary's story as ridiculous, while at others he wondered whether Dubard had really opened the safe in order to investigate. He had been inclined to think that Mary, watching through the keyhole as she had done, had not been able to see distinctly, and that in her limited range of vision had imagined that the safe was being tampered with. Ricci's words on the telephone were, however, ominous. Apparently Dubard was in some mysterious way taking part in that vile and despicable plot which sought to brand Mary as a traitress equally with himself.