One unusual fold stuck boa's side

[Fee]

Hi everyone,

I've owned my Bolivian silverback, Tephra, for about 3 years. He has never had any health issues before, and he still seems to be displaying perfectly normal behavior and no other unusual symptoms, but he has one single large fold that appears to have been permanently stuck folded. It is a longitudinal fold located on his right lateral side, slightly cephalic to his stomach region (approximately halfway down is body length on his right side). It does not appear to be the superficial type of fold one might see during shed; it visually appears to involve deep layers of skin, though not quite muscle. It is shaped as though he has that part of his body draped over a branch or arm, as when a normal snake's skin pooches out a bit to the sides as the snake slides his weight over the branch. Indeed, it looks completely normal when he is draped thusly, though it sticks in that shape even when he moves on. He shows no signs of distress, had a perfect shed a little less than a month ago, and all his tank parameters are normal (90°/75°, 60% and 12 full spectrum, 12 off). He had a medium rat about a week ago. The fold had appeared a few days before that. He has since had a normal excretion. He is just over 4 feet long. The "fold" I'm describing is about four inches long.

Has anyone seen this before?

I know you'll all ask for pictures. My phone is full at the moment but I hope my description is precise enough that someone might have an idea. Thanks, all!