A dark patch is evidence, not a diagnosis. Water may be entering through the roof or wall, rising from a wet base area, or condensing on a cold surface after humid air is trapped inside.
Inspect after rain and after dry weather
Fresh staining after wind-driven rain points you towards coverings, joints and openings. Moisture that appears during cold, closed-up periods may have a condensation component. Check under furniture and at the back wall as well as the obvious window corners.
Fix water management first
Clear roof drainage, keep soil and planting away from timber, and make sure the base does not hold water against the building. Ventilation can help moisture leave, but it will not repair a leak.
Do not coat over damp timber simply to hide the mark. Find the route, allow the fabric to dry and follow the maker's repair and treatment guidance.