Going with the cheapest possible solution first, at the suggestion of the Autozone guy who ran the sensor readings, I've used injection cleaner in the last 3 tanks of gas. There has been considerable rain since and not once has that dash light come on and the car has been running smooth. While it'* tempting to assume all is okay, I'm not going that route. I'm having the car looked at this week for the "Antifreeze Loss" (see other thread) and have printed off all of your terrific suggestions here to give to the mechanic who is going to do the work. (A relative) I'll report back as soon as I know what the skinny is.
Thanks again everyone!