leaving lights on at night
aircraft parked at night, crew leave aircraft live and lights on for the cabin to be cleaned, de-catered, and engineers to do daily maintenance checks, then if it is going to be freezing at night then leaving the lights powered and the doors closed will stop the potable water freezing and keep cabin a little warmer. To drain potable water system in all the water boilers, pipes and tanks takes time, and water on the ramps makes a nice ice rink for the loaders and push back tug the next morning.