How does the letter Sofronia drops lead Don Pasquale to the garden?

In Donizetti's opera Don Pasquale, the letter is a calculated part of the deception orchestrated by Dr. Malatesta and Norina to manipulate the elderly Don Pasquale.12

After a heated argument in which "Sofronia" (Norina in disguise) mocks and slaps Don Pasquale, she flounces out of the room and intentionally drops a letter. This letter contains the details of a secret tryst arranged for that evening between "Sofronia" and an unknown lover, who plans to signal his arrival by serenading her in the garden.2

Upon finding the note, an enraged Don Pasquale is convinced of his wife's infidelity. He consults with Dr. Malatesta, who encourages the idea of eavesdropping on the lovers in the garden to catch them in the act. This leads Don Pasquale to the garden, where he intends to confront the pair and finally rid himself of his troublesome wife, ultimately falling directly into the trap set by Malatesta and Norina.2