Reasons for a disappearance vary widely; misadventure, mental health, abduction, dementia, migration, conflict and more. Approximately 85% of cases in Australia pertain to mental health. About 1-2% are criminal in nature.
In Australia, a missing person is defined as anyone who is reported missing to police, whose whereabouts are unknown, and where there are fears for the safety or concern for the welfare of that person. A long-term missing person is someone who has been missing for more than three months.