My mum found my diary when I was about your daughters age and I had wrote the word "sh@g" in it, bearing in mind pre-laptops and MSN. Well I was mortified because she made me explain it to her, did I know what it meant, what is it etc.
Unfortunately, kids do grow up much quicker these days and whereas before we tended to have our curiosity satisfied by having chats with friends or earwigging into older kids conversations and basically gaining knowledge my osmosis, with the internet and magazines theres just so much more information available.
I read "More" where I was younger and whilst it appeared to be an alright magazine - anyone else remember the "position of the week"? thats kind of what I mean about exposure.
I do however remember going and reading about teen pregnancy, sex, etc when one of my friends at school fell pregnant at 13. Probably would have scared the crud out of my mum if she knew but I wanted to know more, and it was genuine curiosity on my part.
Have you talked to her? I think from the look of it she's either a) been looking up stuff people have mentioned eg nipple piercings, until you've seen one I don't think you'd understand it, thinking back to my 14 year old self I don't think I could have worked out the intricaties of a nipple piercing to be honest or (b) google surfing, you know looking up one thing, it leads you to another search, you look at that it leads you to another site etc and she's just been satisfying her curiosity?
Maybe she didn't tell you because she feared this sort of reaction (and this is not a critism, genuinely) but maybe she thought that she would have been undiscovered and thus no harm caused, or is it possible she was embarrassed to ask you?
On the upside she's not tried to hide it be deleting any of the history either so at least she trusted you that much?