It is indeed related to the dash lighting level. When I adjust the dial, the odometer reads :Ill.1, or Ill.2 thru 6 depending on the light level. It reverts back to mileage after a few seconds.
I still don't know why the odometer suddenly switched the display to the "illumination level". Since I spun the dial one evening last week to confirm this, I haven't seen it switch itself back. I can only guess (as someone suggested) that the dial just happened to be near the piont of changing from one level to another and it was sending the new reading to the display. Certainly this happened in cold weather, but I think this also happened some months ago during summer.
Odd behavior but something I now assume I don't have to worry about.