[People do not normally visit during the day--it's a night, he's noticed, when most people often come to pay their respects--and yet here she is. At "his" grave.
Itachi doesn't speak, not wanting to interrupt whatever communion she is in the middle of. If that is what she wishes to do, then he'll leave her to it. Instead he stands off the side, near a tall tree. He doesn't hide, but he does not attempt to make himself known.
It's like with the boy who was here some days ago. What good does speaking to a grave bring people?]