Breaker Mapping Questions
240v / 30 amp would typically be for a clothes dryer outlet.
As people live in homes, and time goes by, they change things. I'm guessing, of course but possibly the dryer is now a gas dryer and there behind that dryer sits an outlet that isn't being used.

Other stuff :
A outlet in the garage for a "welder" that somebody had installed. And possibly hidden behind something.

Again, I'm guessing.