The tin hat guess. Did you include Google analytics embedded in the pages? Do you navigate to pages and Google analytics sends that data home? 10 years ago I discovered that Google analytics would send the equivalent amount as organic users; meaning if we sent an email newsletter with links to articles, Google would send almost 1:1 ratio the same number of people from search results. They are tracking everything and using it for more than just reporting.

Do you use a CMS or other tools that auto generate sitemap.xml? Perhaps you unknowingly told Google about those sub-pages.