The search bar is only visible in the main site listing view, but not on post pages or during editing. But there's usually plenty of space in the top bar, so I see no reason to omit it.
Please make it universal.
**Edit:** Now, the search bar on the main page is "live", filtering results as one types. On other pages, it should just be a "dumb" search bar, and only upon pressing [Enter] should one be brought to the main page and the search be performed.
The search bar is 'live', i.e. it displays results 'as you type' (with a slight delay of course).
How to you see that working when you are on a question edit page?