Thanks for the thought provoking responses. They were all things that I have discussed with my local mechanic who is extremely good. I also just had a pretty extensive conversation with my dealer's shop foreman whom I trust based on past experience for over 15 years.
The conclusions reached at this point indicate there had to be a loose connection, probably a frame ground, that provided a spark at that particular time to cause ignition of the hydrogen gas put off by a battery that had been charging while driving previously. Just a few minutes before starting this time.
I'll pull the batteries and clean the tray and surrounding area with baking soda and a lot of water. Then repaint the whole mess before installing the new batteries. Of course part of that proccess will be cleaning and retightening all cable connections.
Thanks for all of the input. It was confirmation of everything that I had considered before posting my query, and sometimes that is all we need.