"Service brake assist" fluke?

2014 Volt owner, since January 2018. Love this car!

Recently, in -20° weather, I had exactly one drive where suddenly, while slipping on hard-packed snow, my car threw alerts for "service traction control" and "service brake assist". Then the brake assist actually stopped working! The pedal was very hard to press, but the brakes still worked. ABS light was on though, and no ABS for the rest of the trip. I got to my destination OK and was going to slowly and carefully drive home and take it to get serviced later when the weather was nicer.

But...after coming out of the store, the alerts were gone. The brakes (including ABS) worked fine. I haven't had any issues or alerts since. I have no idea what happened and am tempted to treat it as a fluke and ignore it.

I did recently replace the 12V battery. As far as I could tell, the previous battery was still factory original (so > 8 years old). I had just gotten in my car after work and my car acted haunted, with basically every possible alert sounding, "engine not available", and car refusing to move despite charge on the main battery. I received a jump, let the engine run a while with the hood up, and drove to an auto parts store to buy an AGM battery matching the 2014 Volt in their system. I installed it a couple days later and everything had seemed to be fine after, for about a month before this latest incident.

Any ideas what might have happened? Is it worth checking out at a dealership a 45-minute drive away (I don't trust my local dealer)?