There is no twenty-four hour rule. It is a fiction that has cost lives, and no published guidance anywhere requires anybody to wait before reporting or before acting. A missing person is treated as at risk until an assessment says otherwise, not the other way round.
What is recorded first, because it decays fastest.
- The last confirmed sighting — who saw them, where exactly, at what time, and how the time is known. "This morning" is not a time; "when the school bus passed" is.
- What they were wearing, in the words of whoever saw them, and a recent photograph — one taken this month, not the one the family likes.
- The phone. Number, whether it is ringing or dead, when it last connected, what messaging accounts they use. Ask the family not to keep calling it flat.
- Medical. Medication that must be taken, conditions that affect judgement, dementia, diabetes, epilepsy, mental health crisis, recent discharge from hospital.
- State of mind and circumstances. A row, a debt, a diagnosis, a bereavement, a court date, an immigration decision. Whether anything was taken with them, and whether anything was left that they never leave.
- Who was the last to speak to them, and who has not been spoken to yet.
Treat the home as a scene if there is any possibility of a crime. The most common irreversible error is a well-meaning search that moves everything before anybody has decided whether a crime happened.
The risk factors that change the response. A child. An older person with cognitive impairment. Someone who has said they intend to harm themselves. Someone who left in the clothes they were in, without a coat in winter, without money, without medication. Someone at risk of exploitation. A disappearance completely out of character.
Across borders. INTERPOL publishes Yellow Notices for missing persons at the request of a member country's national bureau; that is a channel between police services, not something an association or a family uses directly: https://www.interpol.int/How-we-work/Notices/About-Notices . Where people are separated by armed conflict, disaster or migration, the ICRC and the national Red Cross and Red Crescent societies run Restoring Family Links: https://www.icrc.org/en/what-we-do/restoring-family-links
Do not publish a person's photograph and details on social media before the service handling the case has asked for it. A public appeal is a decision with consequences — for a person fleeing violence it can be the thing that locates them for the person they fled.
Source. INTERPOL, About Notices, https://www.interpol.int/How-we-work/Notices/About-Notices ; International Committee of the Red Cross, Restoring Family Links, https://www.icrc.org/en/what-we-do/restoring-family-links ; UNODC, https://www.unodc.org/
What this is not. Procedure. Every police service has its own risk assessment and its own thresholds, and where they differ from this page, they govern.