If I understand your question correctly, you state that a diagnosis of carcinoid syndrome is suspected because of symptoms and high urine 5-HIAA, but the CT scan is normal and you want to know what other tests should now be done to prove the diagnosis. Failure to find carcinoid tumors when proven carcinoid syndrome is present is not so rare. This occurs initially in 10% of cases.
First, other findings must prove the syndrome. The various blood markers should be tested to confirm the 5-HIAA significance. This would at least include blood serotonin, chromogranin A, calcitonin, neuron specific enolase, substance P and pancreatic polypeptide; if any of these is positive it would support the diagnosis and then an OctreoScan should be done.