Dogs would definitely be out of the question unless they jumped a 5' fence. The bottom of the fence is buried about 16 inches into the ground. But there are raccoons, cats and even Bobcats. I have never seen evidence of them and at night Opo is in his hide on the screened in porch.I'm not an expert but to me it looks like an injury and probably from a long time ago. Maybe even as a baby. Maybe lately. When I first got Sapphire, he had holes in the top of his shell but his looked like bad nutrition. All but one little sliver has healed over. You can still see a little sliver of hard white though and I've seen the colored part completely closed. So I think it will eventually heal completely. I hope I'm making sense in comparing the two. Can any animals get to him now? Since the time you've had him? If not then it's probably old and maybe healing. But since he's growing it's showing. That's just my opinion. Others are more experienced take their advice.
Thanks Cathie