That is a very weird injury, especially with no evidence of bleeding. Maybe whatever happened was outside, so whatever bleeding happened there? Is he ever in your yard unsupervised or anything like that? But I think you'd still notice some dried blood—I would.
If there was no blood evidence at all, I would think it was just one of those odd things the body does to heal itself. Maybe a very small injury eventually necrotized a larger area, or something.
But at least it wasn't too much ear to lose. I've seen some bad ones. One cocker spaniel I groom is missing pretty much half of one ear leather, a straight line right across. Scared the heck out of me when I started brushing. The owners didn't mention that, and she got normally styled long, full cocker ears, so I really couldn't see it until I was combing through.