I have a switch that's part of a three switch panel, and it doesn't seem to do anything. The other two switches go to a ceiling light in the same room, and a ceiling light in an adjacent room. I've checked most, if not all of the plugs in this and adjacent rooms by plugging a lamp in and flipping the switch, to no avail.
After additionally searching the attic, I've discovered two cables obviously connected to the switch panel. These seem to go to the two lights that are affected by the other two switches. There's a third cable leading from an exterior motion light to an area between the 3 panel switches and the nearby breaker panel. I can't tell which it might be connected to. The motion light doesn't seem to activate in either position of the switch, but I've known those things to be finicky.
How can I determine what the switch is meant to do?