I can't remember if I read it in John C. Walter's wartime memoir "My War" or Truman Smith's "The Wrong Stuff" that briefly mentions Miller's disappearance and mentions that it was believed Miller's plane was accidentally downed by friendly bombers that had to drop their bomb load over the Channel due to being forced to RTB because of bad weather.
Also saw a documentary awhile back about possible freezing of the fuel lines which caused the engine to quit and they then crashed into the Channel and another documentary that claimed Miller was on a super secret spy mission to France and was captured by the Germans, tortured to death and then his body left on the door step of some brothel in Paris. Only problem about the spy theory is that Paris was already liberated before Miller's disappearance.