Brian, you are correct. It is "shell rot" - though it is in the bone. :-(
It looks like she came to us with it from the breeder. The vet cut away a lot of her plastron from two areas on her femoral scute. It looks like it has been under the shell, in the bone, and drying the shell out as it...
I was cleaning my indoor Hermann's habitat today and noticed damage to the plastron - looks like it's on the femoral scute. It's a small round hole - the outside shell is just gone. It's not very deep.
I'm not sure if this is damage from climbing around obstacles in his cage (though I can't...