2010 Prius, ICE sometimes doesn't turn off

Hey all, long time no talk. I used to be fairly active here when I bought my '10, but then things settled down. I do have a question for the crowd, though:

I've noticed now on three occasions that the ICE isn't stopping at stoplights. Let me back up.

I know that the ICE won't stop if the battery is low, I know it won't stop if the battery is too high. I know it won't stop if the temperature is too low. Jeez, I sound like a Dr. Seuss book. I've put 117,000 miles on this car in over 7 years, I know how it feels when it's working right.

When things aren't working right, I'll observe the state of charge indicator dropping down to 1 bar and then shortly going up to full bars, then down to 5-6 bars where it usually sits. Prior to the last few months I think I could count on one hand the number of times it went down to 1 bar.

So after that happens it just doesn't turn off at stoplights. I even left it running in the garage and it ran for 5 minutes and didn't stop. State of charge was 6 bars and the ambient temperature was 77 degrees.

It doesn't do this all the time, just three times in the last month. Is this just a normal progression of how the battery ages? Toyota service ran a diagnostic and found nothing wrong, no codes and the battery seems to be in good shape. But naturally I don't believe them.

Sorry for the long story, but I wanted you all to have the information you might need to help me decide if this is a serious problem.

 

Yeah, the warranty is still in effect, and yeah I've thought the same thing.