Here's something I didn't know - but should have, because it's obvious... Your screen's auto-brightness depends on your webcam. If, like me, you have a privacy cover - this happens:
The MacBook I'm using doesn't have any lux sensors that I can see - most phones have a separate sensor which means the camera isn't in use all the time.
What I find curious is that the camera's hardware light isn't on. It turns on when an app is accessing the camera - but not when the OS is.
Is that a security hole?
Anyway, if you're wondering why your screen isn't adjusting its brightness correctly, uncover your webcam. If you've got nothing to hide...