After the scene with August when he advised Henry to look in the book for proof, the next time we see Henry is him sitting outside Emma’s place with a ring of skeleton keys. I think it’s very safe to assume that Henry was inspired from a story he read (perhaps the one added by August). He says:
Henry: I have proof. [holds up skeleton keys] This is how my mom got into your apartment. This is how she framed Miss Blanchard. Emma: Did you steel these from her office? Henry: Yeah. The book said they could open up any door.
When Emma discovered that one of Regina’s keys actually fits into Mary Margaret’s lock and opens it, Henry exclaimed, “Now do you believe me?”
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"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy