Many damp experts offer damp surveys, and these will be different in terms of diagnosis. Data is king when it comes to damp surveys, that is why I follow the methodology in BRE DIGEST 245 as this is quantitative. All of my damp surveys are invasive, unless it is a pre purchase damp survey. This means aerial surveys, CCTV drain survey, thermal imaging, and gravimetric sampling (bre dg 245) Have a look at the below video that shows how I go about my damp surveys. You will also see the graphs in relation to ‘damp’ from the gravimetrics.
Make sure when asking for a damp survey that ensure the damp survey follows the methodology in BRE DIGEST 245, and takes plaster samples for analysis.