falciform ligament sign
It is almost never seen in isolation as, if there is enough free gas to outline the falciform ligament, there is usually enough gas to also provide at least a Rigler sign.
The falciform ligament connects the anterior abdominal wall to the liver. The ligament continues to extend inferiorly beyond the liver where it becomes the round ligament. Given that the falciform ligament is situated against the anterior abdominal wall, it is not surprising that it becomes outlined with free abdominal gas in a supine patient.