Trace a breaker to what it powers
Note: This may seem silly, as it's just the opposite of what you've already done, but I promise it is not in jest.

Try turning off ALL breakers except for the mystery breaker. It is completely possible that one or more circuits were erroneously "double fed" by this breaker and another.

Alternatively, you could have something not-so-obvious like an exterior light, garage door opener, doorbell that is powered off this circuit.

Lastly, someone could have "dead ended" an unused circuit by simply capping it off where it terminates, but leave the breaker and wiring for future use.