In auto mode, the daytime headlights are (almost) always on. The manual says they turn off if you set the parking brake. I use auto mode and the (night time) headlights always come on when I'm in the garage. It seems to take several seconds after I've backed out of the garage and into daylight before they turn off: I assume that there's some sort of delay logic that keeps the sensor from automatically turning them on/off for temporary changes in lighting. Occasionally they seem to stay on for a few minutes if it's a cloudy day. Once the headlights are on, if you have the "Escort" feature enabled, it will keep them on for a time after you turn off the car.
I don't know for certain where the light sensor is located. I assume it might be in the housing on the windshield in front of the rear-view mirror. But now that I think about this, that might be a problematic location, because it would be exposed to oncoming headlights at night. I wonder if there is a light sensor somewhere in the cabin?
2018 Niro PHEV Gravity Blue