Given its high diagnostic accuracy, 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) has often been used as initial staging and follow-up imaging in patients with malignant melanoma. In the majority of cases, the primary tumour and its metastases show high metabolic activity, and even small subclinical lesions can thus be detected by 18F-FDG PET/CT scanning. In 2010, an important step forward was made in the treatment of malignant melanoma – the era of modern immunotherapy began, and ipilimumab represented one of the first effective drugs. Herein, we present a case of a women with recurrent, metastatic melanoma with persistent increased 18F-FDG accumulation in the abdominal cavity. The lesion could not be identified on exploratory laparotomy and remained unchanged during ipilimumab treatment. Surprisingly, repeated PET/CT examination localized the FDG uptake to an asymptomatic colon polyp histologically corresponding to adenocarcinoma in situ arising in villous adenoma.