So, I got rid of the slough and ended up with this: (I rounded it off better later.)
She had no heat, no pain, no swelling, no lameness, no discomfort -- nada. There was indication of a small bruise about 1/2 down the bar toward the outside. During the trim job, she kept smacking her lips as usual, maybe even more. When I was done, same deal -- no discomfort whatsoever.
I think this is telling me that I have not been keeping the heels far enough down/back so the hoof is taking care of it for me. But, it still strikes me as odd. Other horses whose heels get away don't do this. Why mine? Lisa has been here for 3 years and this is the first time for her. Jade has been here 1 year and she's had it twice.
I HAVE walked the paddocks endlessly looking for something, but haven't found anything except a few sharp rocks.