Many times a mystery switch by the front door is for a future outside post light. Check and see if there is a box in the ceiling in the basement below the switch, easy to get to instead of trying to get into the wall later on. For stuff in bedrooms those are often for future outside flood lights so if you hear a noise outside in the middle of the night you can flip on lights. Check the attic above the switch or see if there is a box in the eaves with no light on them.
