If he had been there before, it might not explain knowing the book’s location. That he must have learned by spying on Henry. The book would only have been in Henry’s hideout in the last year or so, as Henry was not given the book by MM the moment he got to SB, but only when he was in her class.
I do wonder if he could have gone and come more than a few times without Regina connecting the dots on him. Even if she didn’t know who he was, if he even showed up 2 times I think she ought to have been written as more suspicious and wary of him, more freaked out, than she seemed to be. She really felt quite threatened and was confrontational with Kurt the moment they sort of met. She calmed down for a bit but then, of course it ended tragically.
“If I had a world of my own, everything would be nonsense. Nothing would be what it is, because everything would be what it isn't. And contrary wise, what is, it wouldn't be. And what it wouldn't be, it would. You see?” -- Lewis Carroll, Alice's Adventures in Wonderland & Through the Looking-Glass