Sherlock’s heart stopped for a second. No. John couldn’t be here. He couldn’t know where Sherlock was. Sherlock has been chasing Moriarty for about three years now, and only Mycroft knew that he was alive and where he was.
John didn’t seem to notice his presence. He seemed to be lost in thought, looking into his journal. He looked good. Somewhat older and thinner than Sherlock remembered.
Sorry, it was what I thought when I saw this scene for the first time. Sherlock looked like as if he had seen a ghost. A ghost of John. It reminded me of some fanfics in which John dies and becomes a ghost.