How does Marguerite learn of the Scarlet Pimpernel's identity?

Marguerite discovers that her husband is, in fact, the Scarlet Pimpernel completely by accident.
One morning, Marguerite is expecting a visit from her dear school friend: little Suzanne de Tournay. Having nothing else to do but wait for Suzanne to arrive, she goes downstairs and, after a moment of hesitation, decides to go to her husband's apartments.
Once there, she enters her husband's small study, which he always keeps locked and hidden—the only person who is allowed to enter it is his valet, Frank. Fortunately, Frank is busy elsewhere in the house, and Marguerite is left to freely explore Sir Percy's sanctum.
She discovers that the room is kept very clean and organized and that Percy keeps two maps of France and a large portrait of his mother inside. Thinking that there's nothing more to see, she turns to leave and accidently knocks her foot against a small, golden ring which is engraved with a small, star-shaped flower: the device of the Scarlet Pimpernel.
Thus, Marguerite uncovers the true identity of the Scarlet Pimpernel.