Why does Barbarina search for a pin in the castle garden?

In the opera The Marriage of Figaro, Barbarina searches the castle garden for a pin that she lost. This pin was given to her by Count Almaviva to deliver to Susanna as a token confirming their planned tryst.1

The pin had originally been used by the Countess and Susanna to seal a note from Susanna to the Count, which requested a meeting in the garden. When the Count received the note, he pricked his finger on the pin and dropped it, later entrusting it to Barbarina to return to Susanna as a signal of his agreement to the meeting.21