A 3-year-old boy presented with intermittent abdominal pain, melena, and anemia since 2 months. Upper gastrointestinal endoscopy did not reveal any significant abnormality. The child was then referred for Tc-pertechnetate scan to rule out the presence of ectopic functioning gastric mucosa. Tc-pertechnetate scan revealed abnormal tracer accumulation in the thoracic region, which persisted in the delayed images, suspicious for being ectopic gastric mucosa. Subsequently, ultrasound and contrast-enhanced CT (CECT) scans of the chest revealed a large multiseptate cystic lesion in the retrocardiac region, corresponding to the site of increased tracer uptake on Tc-pertechnetate scan.