Once in a while you come across a room with a mystery wall switch. You flip the switch up and down and can't quite figure out what it controls. If you have electrical training you may be able to follow the wires behind the wall, but there is a cheaper and easier way to solve your mystery. Use a low-tech method to figure out what your switch operates.
Things You'll Need
Look around the room in which the mystery switch resides. Turn on all the lights in the room, to make sure that they have good bulbs. Do the same with any lights in rooms adjacent to this one, and outside if the room has any outside walls.
Replace any burnt-out bulbs you find with fresh bulbs. Turn on the lights again to make sure that the bulbs are good. Turn them back off again for the test.
Bring a table lamp with a good bulb into the room from another room.
Turn the light switch to the "Off" position. Plug the table lamp into the outlet closest to the switch. Turn the lamp to the "Off" position whether the bulb is lit or not.
Flip on the light switch. Check all of the lights in the room and adjacent lights. Switch the lamp to the "On" position. If you find a light that is on, you have solved the mystery. If not, unplug the table lamp and plug it into other outlets in the room until you find one that activates the lamp.
If none of these methods work, you may have faulty wiring in your switch or a miswired switch. Consult an electrician to make sure.
If you have a gas fireplace in the room, this may be the switch designed to work the optional fan in the fireplace bottom.