Her diary was mentioned as early as in the ep Angel, when she thought Angel had read it and freaked.
I don't thjink it was mentioned after Ted. I would be surprised if she dared to write after that. I'm suprised she dared to keep on in the first place. Considering what we know now, that she went to a asylum because of her "fantasies", it's weird. But I doubt that was in Joss plans for Buffy in season 2.
Sidenote, in the non-canon comics pre-season 8, I think I recall Dawn finding Buffy's diary and showing it to their parents and that's how they found out. Seems like a bad choice plotwise for the comic, since Buffy still had the diary on the show. If that was how they found it she would most likely not have it on the show. Which makes me think that they definitly found out another way.
Back on topic; It's weird that she kept the diary, no matter how they found out before, and that the asylum idea probably wasn't though of yet, since if someone had found it, it would mean trouble.