In this case, it sounds like the both of you had some mutual comfort in how she dressed, in that both of you trusted both of you to not make a big deal of it. Maybe she thought of it this way and maybe she didn't, but it seemed like she basically trusts you're not too creepy to wear a skirt around without disastrous consequences. She's not going to hold it against you for "taking a peek." But, what *will* make her panic is if you take it as an invitation (as if she's "asking for it"), or if you publicly shame ("slut shame") her for putting you in such a difficult position.

**The way to not be creepy is to not be a threat.** Taking a peek has nothing to do with whether you are a threat to her safety. It sounds like the two of you are basically on good terms.