My father used to have pain in his abdomen out of nowhere with a couple years in between, but they never could figure out what was wrong with him even during an extended hospital stay at one point. I think the earliest time I must have been in elementary school, but my first semester in college during the weekend where my parents were going to come down to visit for the first time, he had the recurring pain again that got so much worse than usual that he was jaundiced and my mother forced him to go to the hospital again (medical situations are basically the only circumstance where I've seen him get irritable), and it turned out that his gall bladder was completely full of stones that they said would have started spilling over into his liver and put that at risk as well if he delayed coming in for a few more hours. They removed his gall bladder, and he's never felt any symptoms in the decade since.