An erect chest X-ray can show even a very small volume of free abdominal gas.
Reference to the clinical setting is required to determine if this is a potentially life-threatening perforation.
This patient had undergone laparoscopic surgery earlier in the day. The free gas under the diaphragm is insufflated carbon dioxide, an acceptable post-surgical finding.
A careful check should be made for free gas under the diaphragm on every chest X-ray.