How can I compare the likely causes without opening the assembly? The safest answer starts by preserving timing, location and trigger information rather than assigning a cause from one clue.
What this question is really testing
ground moisture, rainwater, vapor movement and cooler surfaces can create overlapping below-grade symptoms. The observation becomes more useful when it can be compared with surface drainage, groundwater, capillary transport, humid-air condensation, plumbing leakage and stored wet materials.
A useful next observation
Map wettest locations after dry and wet weather. Record the result together with conditions that stayed unchanged so the comparison remains honest.
Safety and handoff boundary
Standing water, electrical contact, sewage, unstable masonry, extensive decay or inaccessible confined areas require professional controls and should not be entered casually. Provide drainage layout, rainfall history, water-depth or moisture maps, pump behavior, foundation location and ventilation or dehumidification settings.
