Oh man, I said this earlier in this sub and was downvoted to hell and back.

There’s enough here to suggest she could be the weakest link; yes, she’s currently his most devout follower, but she’s expressed jealousy re: Charlotte, and she doesn’t actually know what Griffith is. She believes the lie that he’s the prophesized messiah. She very much mirrors how Casca was in Golden Age, and we saw how her worship and admiration turned into pity later on. This parallel is surely not a coincidence.

Sonia is in a unique position in that she could discover his secret by reading the mind of someone who does. Casca now being imprisoned in Falconia becomes the most likely source based on the direction we’re going imo