Final answer:
It is true that a security guard must not leave their post until properly relieved; doing so maintains continuous surveillance and prevents potential security breaches.
Step-by-step explanation:
It is true that a security guard should not leave his post until he has been properly relieved. This is a fundamental principle in security duties as it ensures that there is always a guard present to protect the property or people, maintain safety and security, and prevent gaps in surveillance. An unattended post could lead to security breaches or other serious consequences. For instance, if instructions specify that a guard should remain at their post under certain conditions and those instructions are not followed, it can jeopardize the entire security protocol.