My brother was the one who first told me about the Light Fairy. Every home has one, he says, and most offices. The light Fairy turns on lights. That's pretty much the Light Fairy's purpose in life.
I don't know how it is in your home, but in mine... well, the lights go on, and they tend to stay on. So we'll be bundling everybody into the car, and I'll realize that not only are all the lights on in the living room and the kitchen, but also in both bathrooms, both of the boys' rooms, and the main hallway. Quite possibly a closet or two, as well.
Now, sure, I might think that the lights were on in the boys' room because we'd never managed to turn them off after getting the boys out of bed. I might think that the lights were on in the bathroom because nobody can finish their business there before Firstborn comes bursting in to see what they're doing. I might think that the hallway light was still on from when I was stumbling around early, early this morning when the cats were yowling for food. The kitchen has excellent natural light, so it's easy to forget that the electric lights are on. Really, there's a perfectly natural explanation for all of this.
Except I know... and now you know, too. It's the Light Fairy at work.