Approximately 600,000 missing person cases are filed in the United States annually per NamUS. Many of these cases or solved or "cancelled" per the FBI.
People may disappear for a number of reasons: physical danger, drug or alcohol use, environmental/weather factors, foul play, disasters, enforced disappearances, mental health conditions, or for unknown reasons.
Amnesty International indicates that this is a significant global problem.
Thousands remain missing.