Just noticed, in panel five, Liz says, "You transform. It's not like you're the only person who does that." She is not hesitant, she is confident. How does she know that? She's right, but everyone's mostly been discrete, it is not being presented as common knowledge.
Charolette's Web forum is a possibility, not sure it explains her confidence, though.
Conversely, in pale six, beyond Liz saying that she does not personally transform, if she had observed someone transform, it seems like she would have mentioned that she had; it's stronger evidence than "You don't seem surprised by the concept". Also, trapping Elliot into that admission does not appear to be planned, she just seems to be fast on her feet.
I'm not getting what this possible Liz mark is, but it's got to be tied to insight.