A missing person was an individual who had inexplicably disappeared. This could later be found to be due to kidnapping, becoming lost, or death, suspicious or natural. An individual missing as a result of combat was said to be "missing in action", and could later be found to have been killed in action.
In 2152, Admiral Maxwell Forrest told Jonathan Archer that the Enterprise NX-01 may have solved the greatest missing person case of the 22nd century, after remains possibly belonging to Zefram Cochrane were located. (ENT: "Future Tense")