Carcinoembryonic antigen (CEA) is a tumor marker most often associated with colorectal cancer, but elevated levels may also indicate other malignancies, including lung cancer. Although not recommended as a standalone screening or diagnostic tool, persistent elevation in CEA levels may warrant further investigation. This case report describes a 71-year-old woman who was found to have progressively rising CEA levels during routine private health screening despite being asymptomatic. Initial workup, including colonoscopy, revealed no evidence of colorectal malignancy. However, further evaluation prompted by new respiratory symptoms and imaging findings led to the diagnosis of Stage IVa lung adenocarcinoma. This case underscores the importance of a comprehensive diagnostic approach in patients with unexplained tumor marker elevation. It highlights the potential role of primary care in identifying atypical presentations of serious conditions.