1. She can hide anything she wants. Shy people always do that.
2. She didn't run away, she just left because she wanted to be alone.
You'd better answer the question: why did Delilah say she knew Ashe's secret? That would be pretty stupid for someone as smart and cunning as this blackmailer.
Like 5 comments before this, I said I don't think Delilah is the blackmailer. So I'm not arguing that point, just asking questions because I think it's really strange you claim there has never been any evidence to support the theory it is her, when chapters 4 and 5 are littered with clues that it's her.
But to answer your question, she told Ashe she knew her secret because she was caught, she slipped by saying she was different and Ashe caught it, and she couldn't come up with an excuse while under pressure.
Two (or three) more questions for you:
1. Ignoring whether she is Ms. X or not and would do what Ms. X is doing or not, do you believe Delilah is intelligent enough to do what Ms. X is doing if she wanted to?
2a. You've said that we now know why she lied about how she found out and that's been settled. Do you think there is a chance she was lying then, too?
2b. What do you think of the coincidence that Delilah has known Ashe was a duplo for months (according to her), but accidentally let slip that she knows to Ashe within 2 days of the blackmail starting?