Our search UI allows users to search across multiple categories: Day, Type, Area, etc. Each selected criteria is ANDed. We also allow a free-text search option, however we expect this to be a rarer use case compared to just selecting 1 or 2 categories (e.g. Dinner on Wednesday).
Initially in the mock-ups we had the free text searchbox as the last element before the search button, to highlight this fact - but it looked visually weird once built in the iOS style - probably because searchboxes usually appear at the top of most iOS table views.
Instead we moved it to the top, and added a placeholder that explicitly says "optional" but I really don't like having to state that - I love a self explanatory UX/UI.
How can optional free text search be represented in a UI like this without resorting to explicit labels/placeholders?
Designer Disclaimer: The nav bar purple is gross I know, still need to fix it since the iOS 7.0.3 update changed the new iOS7 blurred translucency effect.