If a subpage like Blah/Yada exists, typing Yada into the search bar at the top of a page doesn't offer any auto-complete suggestions. I think it should treat the page title as if it were "Yada" for search purposes, since to a human, the page title really is Yada. The fact that it happens to be a sub-page is irrelevant.
Doing a search for "prefix: Yada" also returns no matches, which seems clearly wrong from a user perspective.
I'll file a separate similar task for the new completion suggester beta feature.