A Lady of Rome by F. Marion Crawford

Such a thought could have no real hold upon Maria, and she put it away angrily, as unworthy and ungenerous, even in an extremity which might have excused her for suspecting some innocent person. It was much more likely, she soon told herself, that she had been robbed by some servant in the house, who would sooner or later attempt to blackmail her by threatening to show the letters to her husband. As for knowing even approximately when the theft had taken place, that was impossible. She had opened the writing-case for the last time in May, and nearly eight months had now elapsed. It was one of the objects she had formerly always locked up in a closet in Via San Martino when she left Rome for the summer.

This year she had put it into the deep drawer of her new writing-table, which had an English patent lock, and she had taken the key with her to the country; but she knew that patent locks always had two keys when they were new, and it occurred to her now that she had never seen the second. Since she had been in Rome again she had not even locked the drawer, and had felt safe in carrying only the key of the desk itself. It was impossible to say when it had been opened, and she realised at once how useless it was to waste time and thought in trying to detect the thief.