Something about genies and bottles
There is some information that might help, though it's up to you if you want to disclose it. For instance, how old is she? Do you have a FB account? And so on.
Anecdotally, I remember once accepting a friend request, wondering why that particular person would be friending me, before realizing that I was not in my own account. Absent-mindedness is a parental ploy my mother still uses to this day, pretending that she accidentally stumbled across some information or evidence that she disapproves of. I joke about it with her, now, but ... er, never mind. Suffice to say when I was seventeen she allegedly figured out I was sexually active from three pieces of lint on the sofa, and concealed (buried in a desk, or under the bed, &c.) evidence was lying out in plain sight, visible through an open bedroom door. You could always plead absent-mindedness if you also have a Facebook account.
Or you could try a run-around. We all know that sexual relations change people. Something about her behavior, or in her eyes, or whatever, compels you to ask a certain question, but that's also a patience game; you have to wait for the appropriate stimulus before responding with the inquiries. (Yeah, I know how that sounds.)
And then there is always the painful route, which is to be honest and admit that while you hadn't intended to snoop, well, yeah, that's what ended up happening, and as her father you don't want her to think she's out in the world alone with nobody to turn to for advice or, should difficult circumstances arise, help. This is probably the healthiest way over the long run, but it's also a bit dangerous, like ballet in a minefield. Some parents feel they have precluded themselves from this approach according to longstanding familial custom, but I think you're well aware that reality often changes one's working theories when that reality draws near enough.
Which may or may not mean you'll be revising your moral expressions. There are too many gaps in my understanding of your outlook to be any more specific, but in my world—that is, were I in your situation—the best route is inclusive. As a son, for instance, among the many things that improved my relationship with my father, something had to do with the alleviation of the burden he felt hiding certain aspects of his outlook. Like the time we were having a beer together and I dropped that stupid line I use about masturbation being more gratifying than conjugal relations with the woman who eventually bore my daughter. When I was a kid, that would have earned a certain degree of disapproval. But as adults, it's different. He just smiled into his pint, sipped thoughtfully, then half nodded and half shook his head, saying quietly, "Yeah. Yeah. I know."
That's probably more than I ever wanted to even suspect about my mother, but it could also mean any one of the unknown number of women he had affairs with.
Or when he found out, after estrangement from my mother, that the girlfriend he let move into his place was a meth addict. All I could say was, "Well, now you know what to avoid."
There comes a point when the context of social differentiation between being a parent, to the one, and a friend, to the other, changes dramatically. You never need to know what she does with her tongue while she's down, and she never needs to know your patented hip flourish during copulation. But you'll be of far more value to her if it's not, "How dare you!" but, rather ... er ... okay, I guess it's hard to explain. It's partially instinctive, and depends entirely on the parent and family involved. Like if she comes to trust you enough to tell you about the fight she had with her boyfriend, you can just shrug, look philosophically into the distance, and say something like, "Every guy says that at some point. It's almost like a mystic gate that we have to pass through before we can figure out what's really important."
So if there is any advice I can offer, it is to plot a course that will get you to a place where you can be useful within the scope of her trust.
Which, of course, is nearly useless advice for its vagary.
Good luck with this one.
Considering that I've read her personal messages, I need a reason that wont tip her off as to how I know what they have been up to.
