A damp patch appeared
Order the checks by where the patch sits relative to anything that could be wetting it.
- Is there anything above the patch that carries water?
A pipe, a bathroom, a valley or a parapet above the patch makes an internal leak the first thing to rule out, because it is the only source that is indifferent to weather.
Diagnosing damp in a basement wall - Does the patch respond to rain within a day or two?
That points outside the wall: ground level, a gulley, a downpipe or a path falling toward the building.
Diagnosing damp in a basement wall - Does it have a level upper edge and ignore the weather?
Moisture moving up through the structure behaves that way. Check the external ground level against the internal floor before anything else.
Diagnosing damp in a basement wall - Is the wall wet on the surface only, and worst in cold weather?
That is condensation on a cold surface, and it is a ventilation and surface-temperature problem rather than a water-ingress one.
Ventilation for kitchens and baths - Was the wall repointed recently?
A dense pointing can close the route by which the wall used to dry, so the wall gets damper without any new water arriving.
Repointing mortar and why hardness matters