Primary aorto-duodenal fistula (PADF) is a connection between the aorta and duodenum without prior aortic surgery. We present a case of an 80-year-old female who presented with hematochezia. She was vitally stable but later developed a large episode of hematemesis followed by cardiac arrest. A computed tomography angiogram (CTA) chest scan showed an abdominal aortic aneurysm (AAA) with no leakage or rupture. Esophagogastroduodenoscopy (EGD) demonstrated blood in the stomach and duodenum, but no source was identified. Tagged RBC scan showed massive hemorrhage in the stomach and proximal small bowel. Further review of the CT images identified a subtle PADF. The patient underwent endovascular aneurysm repair but died shortly after. Physicians should maintain a high awareness of PADF, particularly in elderly patients with obscure gastrointestinal bleeding with or without known AAA. Herald bleeding in the setting of an aortic aneurysm should raise suspicion for PADF even in the absence of extravasation on CTA.