Snape Was There

There was a perfect spot for someone to not be seen but to be able to watch them. I'm very confused about like what snape actually there He would have had to have been at least up to the point that harry gets to the lake and sees the sword And then harry ron says he thought he saw somebody You know over there harry does go to look My the what i'm thinking is he stuck around long enough to make sure that all this trouble that he went to actually bore fruit. But as soon as they come up out of the lake and he sees harry's got the sword he says okay my job is done here to separate and then he's gone.