This is a fun question -- I think my zone outs work as memory processing time. I often find myself replaying certain memories, particularly recent interactions I've had with people, and just re-watching the events that transpired in my mind. I sometimes will smile or cringe as I recall these events but rarely do I think about them "critically" (oh, Bob probably asked me that question for such and such a reason!) or have any kind of internal monologue doing commentary over these memories -- I just sort of passively remember them.

It is interesting to think about how this kind of blank thinking time could be used better -- like a rehearsal, as you experience, or thinking in a more directed way. I wonder how automatic this process is -- in my case, will I always by default just re-remembering events? Anyway, thanks for a neat question!