Didn't Remus know that all of the good hiding spots had already been taken? There were several people here at the Sanctuary who had reason to avoid people, and Harry was sitting right at the top of that list. There weren't many places around to hide, even outside (because of the magical barrier that prevented them from wandering too far off from the building), so the thick tree that Remus was approaching had been claimed by someone already. The werewolf, however, didn't know that. He didn't have any idea what he was walking up on, because the tree was serving its purpose and hiding the man standing on the other side of it. It was a good hiding spot, Remus, you'd chosen well... you had just chosen one that had already been claimed.
As Lupin approached the tree, Harry heard the crunching of the fall leaves under his shoes as he approached, not knowing who it was but hoping that they would just walk on by and not notice him at all. He didn't have that sort of luck, though. So when Remus came around the trunk of the tree, he would see the shorter man (the Werewolf had about three inches on him) with his back pressed against the trunk, left hand holding the journal they had all been issued while his right was shoved down into the pocket of his trousers. His hood was up over his head, so until Harry glanced over at the intruder, Remus wouldn't have any idea who he was.
Really, most people wouldn't recognize him at first, anyway (which was what Harry was going for). His hair was only an inch and a half long, he no longer had the scar on his forehead and he wasn't wearing glasses. While his facial structure was the same and his eyes were the same vibrant green, he didn't look the same without the messy, longer hair and those glasses and of course, that scar. The hood being up may actually help Remus recognize him, though, because he wouldn't be able to see that too short hair.
Harry wouldn't have any problem recognizing his former professor, however. While Remus looked tired it was a look that Harry had often seen him wear, so really, he didn't look much different that the Boy Who Lived remembered him. When he finally turned his head to look at the man invading his space, both brows rose slowly and he turned to face him completely, fingers tightening around the journal in his hand. The urge to back away and leave came but Harry shoved it to the back of his mind - he wasn't going to run away from Remus Lupin. Remus wasn't Ginny. The two of them belonged to two separate categories, and while that didn't mean he cared for one any more than the other, they were just two different people on two ends of the spectrum. Remus had been like a father to Harry during school, until Sirius had taken over, and after the death of his Godfather they hadn't been quite as close as they had been during his third year but Harry's feelings toward the Werewolf hadn't ever changed. That was why he stood there and only stared forward at him, instead of running off, waiting for Remus to speak up.
He certainly wasn't going to be initiating this conversation.