I sometimes forget to turn on the headlights during nighttime because in my area of living, there are street lights almost everywhere. it's totally unintentional to not turning the lights on (if it happens)! It's my reason :)
Sometimes, we see a one-eyed car (one sided light is damaged). So, one possibility is that the headlights are malfunctioning. But, it's quite dangerous to drive in a dark place without the headlights on! But as you have said, without asking them directly, we can't know the exact reason!
!BBH
