Finding air under the diaphragm is often a critical diagnostic signal:
Incidence and Significance of Subdiaphragmatic Air ... - PubMed
Subdiaphragmatic air, also known as , refers to the presence of free gas in the peritoneal cavity situated just below the diaphragm. While it is a classic radiographic finding, its significance ranges from a benign postoperative occurrence to a surgical emergency. 1. Clinical Significance