We live in a 1930s semi and have noticed damp patches in the hall. The patches are not large and are low down (approx 30cm up from the floor) are inside the external wall. Their is an air brick near the damp on the external wall. Drill holes along the second level of bricks along this external wall would seem to indicate treatment of a previous damp issue.The house has had cavity insulation but we have lived her for around 20 years and not had this problem before.