Just found this yesterday. This was in a ten year old house just below the master bathroom toilet.
What you are seeing is the bottom side of cement board and the bottom of the actual toilet. There is about a 1.5" gap between the toilet and the toilet flange. What is really amazing is that when I had the client flush the toilet two or three times not one drop of water came out. The third photo is the toilet in the master bedroom above. Not much evidence, even the toilet was tight to the floor because of the caulking around it. The reason the toilet and someone sitting on it didn’t end up in the crawlspace was because the front and rear of the toilet are supported by the joists below.
I couldn’t confirm it but it looked as if there may have been no wax ring installed. The diameter of the damage is about 5 feet and it extends below the bathroom sink, the whirlpool tub and a wall which is shared with the master bedroom. This one is going to cost somebody a lot of money.