Nobody schedules an empty leg. One shows up here because an
operator has an aircraft finishing a trip near Portland and needs it near
the San Francisco Bay Area — or needs it home, and this direction happens to be the way home.
That is why the flight can be a fraction of a full charter, why it can
vanish within hours, and why the return direction is a different flight
entirely with its own price and its own aircraft. Watching the route beats
searching it once.