There is a limited amount of research in the area of missing persons, especially adults. The aim of this research is to expand on the understanding of missing people, by examining adults' behaviours while missing and determining if distinct behavioural themes exist. Based on previous literature it was hypothesised that three behavioural themes will be present; dysfunctional, escape, and unintentional. Thirty‐six behaviours were coded from 362 missing person police reports and analysed using smallest space analysis (SSA). This produced a spatial representation of the behaviours, showing three distinct behavioural themes. Seventy percent of the adult missing person reports were classified under one dominant theme, 41% were ‘unintentional’, 18% were ‘dysfunctional’, and 11% were ‘escape’. The relationship between a missing person's dominant behavioural theme and their assigned risk level and demographic characteristics were also analysed. A significant association was found between the age, occupational status, whether they had any mental health issues, and the risk level assigned to the missing person; and their dominant behavioural theme. The findings are the first step in the development of a standardised checklist for a missing person investigation. This has implications on how practitioners prioritise missing adults, and interventions to prevent individuals from going missing. Copyright © 2016 John Wiley & Sons, Ltd.