Behind the Throne by William Le Queux

The revelation of the truth that Jules Dubard was making a methodical examination of her father's private papers held Mary spellbound.

From where she bent her eye at the big old-fashioned keyhole, she saw that the ponderous steel door had been opened by a key, for it was still in the shining lock. Within that safe her father kept a number of important state papers relating to the army, and quantities of correspondence had, from time to time, been brought up from Rome by official secretaries and he had placed them there for safety.