You can’t always see it but here there is a clear line of discolouration defining where this wall is damp. The red light on the moisture meter confirms what is visually apparent.
The causes of damp are many. This is rising damp. The discolouration and high meter reading are on the lower part of the wall indicating that ground moisture is being absorbed into the structure by a process known as osmosis.
There is no discolouration on the upper part of the wall where the meter readings were almost non existent. If the wall was damp at higher level it would suggest not rising but penetrating damp- perhaps due to a missing roof slate, gutter leak or plumbing problem.