I think they would have been able to tell if he’d had any sort of surgery concerning the organs in his abdomen. Skin removal definitely is a possibility. Though, this would be more likely if he’d had a gastric bypass or some other surgery which would have caused him too lose weight very fast. He wasn’t that old yet so unless he was extremely overweight, I’m not sure if gradually losing weighy by dieting and sporting would have caused the issue of extra skin. But I am by no means an expert on this!
It’s also possible that he had a mole or an extra nipple of something of the kind removed. Or he may have been in an accident or just injured himself at a time. I wish we had a picture of the scar...
I don’t think it’s a surgical incision because they should have been able to tell if that was the case. Most procedures have fixed entry points. Plus as EllieGato already stated, you should be able to see tiny dots around the scar of where the staples/stitches once were.
The fact that they stress multiple times that the scar is “very” faint, makes it seem more likely that it was caused by some superficial injury. Something that didn’t require stitches.
Added for (future) reference: