Bot of background - 15-yo DD, oldest of 3 DCs. DH and I still married, we do bicker sometimes but mostly are fine, no threats of family split.
She didn't have the easiest ride through primary school, as she never really found her people. She had friends, but was desperate for a 'best friend' which she never really got (one girl was a best friend for a while but turned out to be rather unpleasant, which upset DD greatly at the time). Secondary school, again did well academically but not in a close group and a bit of a third wheel.
So, in Yr 10 she changed schools (start of this academic year) to a specialist school which only takes in from Yr 10. So everyone started together, and she has found her people! She has a big group of friends and is often out socialising etc. So far, so good.
However, she was wearing shorts yesterday, and I noticed some scratches at the top of her legs. Didn't really think anything of it, just said, 'Oh, what have you done there?' She said something about the cat scratching her and she'd fallen off her bike (true). She also immediately tugged the shorts down to cover the marks.
But it's just not ringing true to me as I think about it. The scratches were right at the top of both of her legs. parallel to each other and just the right size to be a single-blade razor pressed into the skin. DH briefly saw the scratches when I asked about them and agrees that they did not look accidental to him either. What do we do?? He thinks we need to watch and wait, and see if we notice any other marks now that we are on the alert for them, and warmer weather means she'll be wearing fewer clothes. I rarely see her without her clothes on these days (naturally!) but he was wondering if I could engineer to catch her in semi-undress to see if there are marks anywhere else. He is also thinking that perhaps it was an experiment (the marks all looked the same age, as it were ) and we could make things worse by drawing attention to it if it's a one-off. I want to rush upstairs, grab my darling darling daughter and talk to her until she tells me whats going on. I've been awake since 05:30 fretting, I cannot believe she is in such distress that she needs to do this 
What do I do?