I've seen other similar questions (How to trace mystery switches), but I have some more details:

  • This is in a master bedroom
  • There is a 2 gang box right near the inside of the door - one switch (purple wire and yellow wire) controls the overhead light. The other switch (purple wire and orange wire) is the mystery switch.
  • There are no switched outlets (I tried with a lamp)
  • There are no outdoor lights that I can notice (as suggested in the other answer)
  • I bought a Fluke Pro3000, turned off the power (purple was power anyway), hooked the red alligator clip on the toner to the orange wire, and tried sweeping the wall with the probe, but it was inconclusive at best
  • The pipe the orange wire enters from the box goes up

Any new guesses or suggestions of other experiments to do?

  • Questions: where in the world are you? Is this a house, and do you have an attic? (Side q: is there a fan in the attic?) If there's an attic, did you try the toner there? Did you use the toner at the panel? – Aloysius Defenestrate Feb 12 '19 at 3:33
  • @AloysiusDefenestrate Chicago, IL. Yes, it's a house. There is an attic, and I went up there but there is a foot of insulation so I can't tell where to step let alone see any wires to tone. The only thing I know of in the attic is a furnace. In fact I got really excited because there was a box near the furnace with an orange wire (and red and white), but it didn't tone out. I know which circuit the switch is on (the same one for the overhead fan/light) - I assume that's what you'd be looking for at the panel? – David Doria Feb 12 '19 at 3:37
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    Chicago has a reputation for running wire in conduit, which makes toners tough. I'd still try and wander around the attic with the toner wand. (Gloves, mask, and crawl from joist to joist -- they're usually 16-24 inches apart.) There's always the possibility that in the past there was a ceiling fan with separate circuits for light and fan... have you opened the light fixture? – Aloysius Defenestrate Feb 12 '19 at 3:43
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    Did you try both sockets in each outlet? Some outlets are split with separate control of each socket. I work a lot in conduit and think the conduit is great news. You can shove a fishing tape down the hole and get a sense of which way it's going, then open up boxes in that general direction. – Harper - Reinstate Monica Feb 12 '19 at 4:57
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    @AloysiusDefenestrate You know, I have NOT opened the light fixture, but in hindsight that sounds like a pretty obvious thing to check. There is a fan + light where the fan is controlled by a remote, so perhaps it's not original and there used to be a fan that this switch controlled. I'll look tonight and report back. – David Doria Feb 12 '19 at 12:56

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