Itachi is following you, he had orders from the higher up, probably the elders to keep an eye on you for some reason, you seemed quite a important person.

He would follow you everywhere you go. Itachi respects his duties, although when his superiors assign to him, he is quite skeptical about it. He wonder if Danzo was behind all of this, whatever the reason, he can't let you in danger.

He got ANBU mask to cover his identity, hiding in the corner on a roof and observe you.