The other light switches aren't mysterious at all. This is what they do.
So. I was thinking the other day, that it was funny that not only did I have a mysterious light switch that I didn't know what it did, and that I also had a light (on the cooker hood) that sometimes didn't turn on when I wanted to. I must have been very tired, because when I decided that these two facts might be related I thought it was a breakthrough!
So I did an experiment.
here is the cooker
here is the cooker with the mysterious 3rd switch on!
So. The mystery was solved : the mystery switch controls the light switch that the cooker light is on. Sorry for such a dull answer. Next week, we investigate what is up the mysterious hatches in the hall and the corridor outside!