Which of the following is a possible sign of hidden mold?

A possible sign of hidden mold can indeed be a smell that is musty or earthy. When mold grows in hidden locations, such as behind walls or underneath flooring, the spores can emit odors, which are often the first indication of a mold issue before any visual signs appear. This smell is produced by volatile organic compounds (VOCs) released by mold as it thrives.

While high humidity can create an environment conducive to mold growth, it does not serve as a direct sign of existing hidden mold. High humidity levels could potentially lead to mold growth, but they don't indicate its presence without further evidence. Discoloration of walls is a more visible sign, typically indicating that mold is actively growing, rather than being hidden. Similarly, cracked paint may suggest moisture issues, which could precede mold growth, but does not directly signify the presence of hidden mold itself.

Thus, the characteristic odor associated with mold is a clear indicator of its possible existence, aligning with the nature of a hidden mold problem.