What the Curate Saw (no relation to the 1906 humorous film short of the same title) is an adventure-mystery written (I suspect) in the immediate postwar era. Its female protagonist, Vivian, seeks to solve the mystery of the pinholes in a cracker (in my dream-recollection, it was more like matzoh bread); when deciphered, the pinholes gave an obscure phrase or two and some locations in Zurich. There was a small boy involved, and an older boy that Vivian was in the process of falling in love with during the novel. I hadn't read (or so I remembered in the dream) far enough to find out how Vivian and company (and I suppose the titular curate) get from the English countryside to Zurich, or even if they ever do.
Oh, I suspect that what the curate saw was Vivian's blossoming romance, not anything to do with Zurich-focused spies.