Mechanics call this the money light. :)

Indeed. When I took my car in last winter because the light had come on for no apparent reason, as the car was running fine, they charged me $140 to take it out on the road to try to find the reason. No reason was found. Two weeks later, the light came on again (towards the end of 2022). The car was and is running fine. The light will remain on until July when I take it in for a scheduled oil change.

Or you could buy a $20 code reader and see what's causing the light yourself.

But the mechanics using their professional grade code reader and related equipment couldn't figure out what's causing it. At least I'm comfortable with "unknown" after that's their diagnosis; I don't think that would be the case if I did it and got that result.

Sounds like it was an intermittent fault? If so, those are notoriously hard for any mechanic to diagnose. Cars _usually_ store code history but it's up to the manufacturer and often the data they provide to the mechanic with a scan tool is misleading or an outright pack of lies.