One thing I want to mention that I'm not sure you're implying or not, is that the girl shouldn't be looking at the camera in the first place. It's supposed to be a secret shot, and after you successfully take it, she doesn't react to it. I feel like her not looking at the camera should be a pretty clear change, from both a logical and a gameplay standpoint.
This is the case for most photos, but there are a few where the character “side eyes” you (like she is suspecting that something is off) and some very rare photos where they are straight up looking into the camera and even flashing.
But you are totally right that the majority of cases are supposed to be pictures of characters that are unaware of the photo being taken.