A 25 year old male had pain in Rt side of abdomen from five days. He had tenderness in rt iliac fossa and lumber regions .X-rays of abdomen showed small free gas under rt dome of diaphragm. USG abdomen showed oedematous thick appendix and fluid around it. On operation perforated retrocaecal appendix was removed with 200 ml pus.

Patient is improving. How often free gas is detected?