Most of us inspector types do not think of error probablity. A few of us who have a better unstanding of human nature do. We all do our best to perform to contract and process procedure and we do miss things and report false positives. As long as we rely on our eyeballs and finger tips we will error.
Variability is normal, especially for humans.
I have never heard of a NDT contract stating that one must find such and such a discontinuity in such and such an area with a .999997 probablility of success. Chances are good that the cognizant engineer does not even have the remotest idea what the probability of a specific flaw existing is a finite matrix actually is much less the NDT contractor. So, does the vendor inform his client that their engineers cannot properly define a scope of work or does the vendor perform to contract. Anyway, there are two sides to the ethics issue and regardless the contract rules. Please click the button only once.