As he mentioned towards the end of his passage, one of the differences he states is, “1. Signs are what a doctor sees, symptoms are what a patient experiences.” if i might add that when he means what a doctor “sees” also means what is smelt, felt, tasted(in rare cases of diagnosis), hear, and sees or the 5 senses. This is the difference that he is talking about when differentiating between a sign and a symptom. A simpler way to word this is that it is all observed by perspective. A fever can be felt and seen by a doctor making it a sign but it can also be a symptom do to the fact that the patient also experiences the fever. So you are right, but technically it is not a special case because there are also other symptoms/signs that fall under this category.
In most cases, CT scan is not used to detect gallstones, but this imaging test does have its uses in the biliary system. First of all, the entire main duct can be seen using CT scan because unlike ultrasound, air in the GI tract does not interfere with CT. High-speed CT with computer-assisted reformatting capabilities allows the radiologist to move quickly through numerous images. The ability of CT to find stones in the common bile duct approximates ultrasound. In general, CT scan is a better test for more complicated problems, although it may be used together with ultrasound.