uhhmmm.... any outlets in the garage???

i had one buried in the corner.. within a few feet of the panel ... which i buried with storage ...

no clue why it was there ... as the rest of the garage had its own circuit..

often wondered if it was done by a prior owner ... before he electrified the the rest of the garage ... he seemed to have done things.. his own way ... lol ... [like the house being T111 ... and a decade later .. vinyl sided ... leaving mysterious posts on the deck ... as the rails previously attached to the T111 .. only took me a few years to figure that one out.. lol ]

ken

ps: what about the attic????? or crawl space ... got a sump pump????

I was at a new house (wired by others) with the same situation. And could never find any load supplied by that circuit. The question may never be answered.

Is it the circuit for smoke detectors?

It doesn't have to be a light or receptacle.
Try leaving it turned off. Maybe you'll notice something that isn't working.
Or try turning all of the other circuits OFF and look for something that is working.

Is there a lighting circuit in the attic?

Does it run the dishwasher or disposal? Maybe the ignitor for a gas appliance?

Maybe just plain abandoned. Just leave it off, and one day, when you have an explicably dead something...

Furnace? Range hood? AC air handler? Could be any number of things.

Are you sure there is a wire connected to the breaker?
It may have been left as a spare or was intended for a circuit that was never installed.

Yeah, leave it off. If it is the sump pump you will know soon enough. Attic fan? Deep freeze in the garage/basement? Attic lights? Doorbell transformer? Dishwasher? Exhaust hood? Minibarn power feed?