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This report presents a case of a middle-aged man who was incidentally diagnosed with a gallbladder (GB) mass. Though he had some features of Cushing syndrome (CS), including those due to mineralocorticoid effects (persistent hypokalaemia), the classical features were absent. Abdominal CT scan showed a large mass arising from the GB with liver and lymph node metastasis. Biochemical evaluation confirmed adrenocorticotropic hormone (ACTH)-dependent CS. A biopsy from the mass revealed a poorly differentiated neuroendocrine carcinoma. The patient was started on ketoconazole and appropriate management for other complications. Chemotherapy was poorly tolerated and hence discontinued. The patient eventually succumbed to his illness. This case illustrates how the absence of classical Cushingoid features can mask ectopic ACTH secretion, with persistent unexplained hypokalaemia serving as a key diagnostic clue in our case. It also emphasises the difficulty of identifying a neuroendocrine tumour arising from a rare, unsuspected primary site.