The title and text content of custom pages are automatically indexed for search. Custom pages are returned when doing a Global Search.
To enable the Global Search bar on a custom page, go to the Edit view of a custom page, and make sure the Hide Title Bar field is unchecked.
When that is the case, the Global Search bar will appear on the top right of the custom page.
If you are actively using Cross-Content Pages, and prefer for HTML Custom Pages to not be returned in search results, you can exclude these under Admin > Settings > Custom Pages.