Final answer:
A certificate of fitness holder should treat an unmarked gas container cautiously, refrain from using it, and contact the appropriate safety officer or supervisor for proper handling.
Step-by-step explanation:
When encountering an unmarked gas container, a certificate of fitness holder should treat the situation with caution. Gas containers should always be properly labeled to indicate their contents, potential hazards, and handling requirements. If the contents of the gas container are unknown, the individual should refrain from using the container and contact the appropriate safety officer or supervisor in charge to handle the situation appropriately. It is important never to make assumptions about the contents of an unmarked container, as it can lead to incorrect handling and potentially dangerous situations.