Good news with regard to my car!  

Taking a break from sweeping the sections of the porches currently in the shade, I got out the owner's manual to track down the fluctuating gauge.  No surprise to me that turned out to be a bit of a scavenger hunt.  

I started with "dashboard" in the index, and that page sent me to "instrument panel".  At first glance it seemed to be the "security system indicator".  But upon further observation when I went to that page, it turned out to just be the tiny light at the bottom of the gauge, with (naturally) no other label for that gauge.  

Several pages later there is a better diagram, finally indicating that 

the red "arrow" belongs to the turbo boost meter.  

No idea why it was swinging up and down, 

but at least I can stop worrying about a dead battery, right? 

Now that I'm reasonably cooled off I'm heading back to sweep more of the porches.  

After that a dip in the pool sounds like a stellar idea!