It's not me who wrote the GIUSP. In truth there are many grey areas within the GIUSP which may lead to complications due to objective interpretation. For example; as you wrote, how would "you know if its drawing in POC" or not if you didn't do a risk assessment? How would you know when to suspect it? What if the terminal was 200mm from an opening and the check was carried out on a still day? You classed it as NCS because you didn't consider it a danger. If the check had been done a day earlier when there was a strong wind blowing in the direction of flue to opening suddenly it's dangerous. Personally I would take away this grey area and do away with NCS, I would still have three classifications ID, AR and SAFE to use.