In order to be on the official Unified list, a school has to hold auditions at all the Unified cities. There are many more schools who only go to, for example, NYC and Chicago, and therefore are not an official Unified school. Don't confuse a school that accepts walk ins with these schools that aren't "official" Unified schools. They are very different creatures.
You can't really know before hand which schools will allow walk ins at Unifieds. They only do it if they have empty slots. Some schools don't. At NYC Unifieds last year, we noticed walk in slots available at Penn State, Ball State, University of Arts, and the Royal Conservatoire of Scotland. I'm sure there were others.
However, my D auditioned at CMU and CCM on the day BEFORE Unifieds officially started. These were not official Unified schools, nor were they walk ins. She applied, made the audition appointment months and months in advance, and auditioned for the schools at Pearl Studios in NYC. Neither of these schools had walk in slots available.
So, the bottom line is, if you really like a school, do NOT rely on walk ins to audition...you may not get a walk in slot. As others have pointed out, you have to call the schools and/or check their websites to see if they are holding regional auditions in Unified cities at the same time, or immediately before, Unified auditions.
Hope this makes sense...?