Greg wrote:

Are you sure it doesn't control half of one of the duplex
receptacles in that room? It might also have gone to the fan in
a fan/light combo that is now just a light. Is it near a
basement or attic access? How about an outside outlet? I had a
"mystery switch" that controlled a landscape light outlet in the
yard. That took a while to figure out. ;-)

Yep. If it's an older (50+) home, it could go just about
anywhere, be broken, or some combination of the above.
Oh, joy! g

If the home is more "middle age" (20-50) or newer, I would lean
towards the 'half-of-duplex-for-table-lamps' as more probable,
and it it's 20 I would think it could have been put in for
"future use", which builders didn't really do until recently.
If so, and it is anywhere like Greg said, you may want to look
for a blank cover plate on an outside wall where you might place
a light fixture or outlet.

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