usual causes, lead flashing needs repair, or chimney is no longer used and condensation build up is causing it, if its not causing damp in your house then no need to worry, I would inform the neighbours anyway
The flashings were replaced a year or so ago so maybe this is the result of poor flashing from years ago. For some reason there is a damp pad that comes and goes in the hallway lower down the same wall. Do you have any idea what this could be?
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