It was probably found by the radiologist. It might not have been obvious to the ER doc as they are experts at emergencies but not so much at interpreting x-rays as radiologists. If this was an isolated occurrence of abdominal pain they may let you choose to see what happens next. I had no gallstones when I had my gallbladder attack and just sandy stuff in the gallbladder so I managed mine with diet. I wonder why yours is swollen. Perhaps you have a stone stuck in a duct? Let us know what you find out.

Feel better soon.