In order for someone to eavesdrop, they have to be in the vicinity of the conversation. Benedick was, in fact, near the conversation of Don Pedro and Leonato. He was hiding behind some trees. This whole conversation was planned out by Don Pedro. He intentionally had Leonato talk to him about his own niece, Beatrice. Knowing that Benedick was hiding, in Act II scene iii, Don Pedro talks to Leonato in a loud voice about Beatrice's love for Benedick. He says it in a loud voice so that Benedick can hear every word. This was just so that Benedick would admit his love for Beatrice. While the two men were talking, Don Pedro asks Leonato if his