How does the author use foreshadowing in Magyk?

At the end of Chapter Twenty-four, Silas leaves in search of Simon, and Sarah asks him to be careful. Simon tells her not to worry and promises to return with Simon soon. The last sentence in the chapter notes that Sarah is worrying, and that Silas doesn't return with Simon as quickly as he led her to believe. This foreshadows that Simon will not be found quickly and that something evil has befallen him.