An 83-year old man presented acutely to the emergency department with generalized weakness and subjective fevers. A month earlier he had undergone resection of a large intramuscular sarcoma from his thigh. The cancer staging work-up was still underway and a decision about adjuvant therapy was still pending. Although initial laboratory assessment showed leukocytosis, this normalized soon after admission without the use of antimicrobials. No fevers were documented. During the admission an 18F-FDG PET/CT was performed in continuation of his sarcoma staging workup. This revealed unexpected abnormal radiotracer uptake in the left sternoclavicular joint with fluid collections extending into the sternocleidomastoid muscle and the mediastinum. Imaging findings were consistent with septic arthritis and abscess formation, despite lack of fever or localizing symptoms. Ultrasound-guided aspiration revealed purulent fluid that grew Moraxella nonliquefaciens. Given the unusual presentation, ongoing clinical uncertainty about the true cause of the septic joint, and concern for an occult sarcoma metastasis, surgical debridement and resection of the joint was carried out. Pathology and microbiology evaluation confirmed septic arthritis with osteomyelitis and abscess extension into the mediastinum. No tumor cells were identified. Postoperative course was complicated by hematoma, but otherwise the patient responded well to antimicrobial therapy.