I have a friend's 2019 Audi A8L for the summer, while he is in Michigan.

Today I went to start it and got the Christmas light dashboard and the engine didn't start.... then it wouldn't turn off.

I had a lunch appointment so took the X5. When I came back the Audi was still on :/

Turns out it was probably the CR2032 battery in the key fob. The old battery was at 2.8V. No low battery warning, though Google says there should have been.

I left it on the charger for the afternoon and it did a full reboot when I started it again this evening. At least most of the settings like phone connections are stored in NVRAM.

Modern vehicles!!