Don't quote me, but I think there was an extra outlet space already installed.
The night light turns on when the sensor doesn't detect another light source.
If you look closely, you can see how the sensor is currently located at the bottom right corner. BF unintentionally installed the light upside down. He figured the vanity light's rays should have reached the sensor. The night light is always on.
Oops. BF will switch it around whenever he's done with everything else. You know, when he has nothing else to do. At least it works...all the time.