I've come up with a theory.
Hero’s whereabouts are unrevealed during the play. However, Borachio does mention that he suspects he will have to lure Hero away from her apartment for the plan. How he shall is unexplained. In Act II Scene 2 Borachio says he'll fashion some way to have Hero be away, but it's not specified how/for what. It's up to the individual production to justify it, but no explanation is given. "...for in the meantime I will so fashion the matter that Hero shall be absent; and there shall appear such seeming truth of Hero's disloyalty…” explains Borachio. Put simply, we do not know where she was, but the text provides the hint Borachio kept her occupied, perhaps by the fashion of trickery. It must have not had her wander around Messina looking for any individual or had her see any individual or she would have met them and whoever she accompanied spoken up during the wedding. Whatever she was told to do she had to do it alone, if Borachio did make up an excuse to get her to leave the house. Another theory states that Hero’s absence was not caused by Borachio’s trickery directly: He may have never come face to face with Hero after all. It is possible that Hero was gotten out of the house by Margaret. Margaret is a close friend to Hero, and she would easily have been able to sway Hero into leaving using this influence.