Which sign may indicate that a building could become unstable?

The indication of an interior collapse serves as a direct warning that a building’s structural integrity is compromised. This can occur due to various factors such as damage from fire, water infiltration, or foundational issues. When signs of an interior collapse are present, it is crucial to assess the situation carefully, as this suggests that the building may no longer be capable of supporting its own weight or resisting external forces.

In contrast, while flashing lights may imply activity or a warning situation, they do not provide specific information about the structural stability of a building. Open doors can indicate various conditions, including that a building is inhabited or accessible but do not, on their own, signify instability. Strong winds can affect structures, but they are more of an environmental factor rather than a direct assessment of structural integrity at that moment. Therefore, the presence of an interior collapse is the most definitive indicator that a building could become unstable.