Sponge-like hypodense lesion simulating fecal matter outside the bowel with surrounding soft tissue thickening and inflammatory reactions in a postoperative patient denoting retained surgical gauze (missed towel). 

When the lesion seen inside bowel, the diagnosis of missed towel becomes difficult as the lesion simulate fecal matter inside bowel, but the presence of air foci outside bowel and surrounding inflammatory reactions suggesting perforated viscus. This makes the diagnosis of a missed towel eroding the bowel wall more probable.

In this case postoperative data confirmed the diagnosis of missed towel eroding the wall of sigmoid colon.