Final answer:
The train operator should report the incident to the authorities immediately when a customer leaves a train without permission, as it is a matter of safety and adherence to protocol.
Step-by-step explanation:
If a customer leaves a train without permission, the train operator must report the incident immediately to the relevant authorities.
This is a safety measure that helps ensure the well-being of the individual who has left and also maintains the safety and order within the train's operations.
Depending on the protocol of the rail company and the jurisdiction, specific procedures such as notifying the conductor, completing an incident report, or stopping the train to ensure the person can be assisted or questioned by authorities may need to be followed.