Chapter 1, Page 37

Lily also considered Little Trumpet Boy being the thief

While drawing that first panel I thought to myself "Why did I draw her that way... Am I trying to make it a SURPRISE that Lily is there? Am I implying she's evil? Huh. What if it turned out Lily was also planning to steal the Gumdrop and she'd been undercover for years! And THIS was the week she was gonna start HER heist! And Hiccup ruined it!"

Then I was like, "Darn, that's a much better story than the one I actually wrote... Oops!"

Ain't that just the way.