My auto's warning system has been calibrated, and the tires filled, so now everything is where it is supposed to be. The timing of that dashboard warning is telling. I was reminded by an acquaintance that the change of seasons brings on a sudden temperature change which can affect tire pressure equally suddenly. So, although the tires had enough air in them anyway, there would have been a sudden overnight change which might have been enough to trigger the dashboard warning. Anyway, it's done now.