Which of the following is a sign of pressure build-up in containers?

Activated pressure relief devices are a clear indication of pressure build-up in containers. These devices are specifically designed to release pressure when it exceeds a predetermined threshold, preventing potential hazards such as explosions or ruptures. When a pressure relief device is activated, it signifies that the internal pressure of the container has risen to a level that exceeds its safe operational limits, thereby necessitating the release of that pressure to maintain safety.

In contrast, signs like rust, normal temperature readings, or the presence of corrosion might indicate other conditions but do not directly signify that pressure is building up. Rust and corrosion could be signs of degradation or exposure to moisture, while normal temperature readings would suggest that the container is not overheating, which is typically related to pressure conditions. Therefore, the activation of pressure relief devices is a direct and critical indicator of pressure build-up.