I know that feeling of not being able to tell which gender someone is from behind or other angles. It's kind of easier in the military, with regulation haircuts and stuff.
I think this is a reference to the "Chamber of Time" game, as I just completed a side quest that is about this exact situation:
Hannah can't seem to get her mind off a person she saw around town in Blytonbury. After Akko gets some photos of Amanda crossdressing (in the above outfit, taken with Amanda's permission) for a different side quest, Hannah happens to see one of the pictures and asks Akko to tell her who it is, as it seems to be the same person. Fortunately, Akko is up front about the issue and tells Hannah that it is Amanda in disguise "as a boy". Hannah's reaction would be best described as "aghast" (screenshot): https://steamuserimages-a.akamaihd.net/ugc/965344033864005834/194F610D515C843611A7950B18B76368CC07D0CE/ Hannah then tells Akko to forget about the whole thing. I guess that could have gotten AWKWARD if Hannah had found that out herself. XD
