Answer: Rigler’s Sign - ie double bowel wall sign

It is the presence of pneumoperitoneum that allows one to see the outer wall in addition to the inner wall of air-filled bowel. I’m sure everyone noted the free air in the left upper quadrant which yields the Rigler’s sign in this case, but in the supine frontal abdomen X-ray, free air can be extremely difficult if not completely occult. A Rigler’s sign may be the only clue to the presence of free air (think recent surgery, perforated viscus)

Reference: http://radiopaedia.org/articles/rigler-sign-1