It took a bit, but I bashed out the brick that was in the door sill, and discovered that the previous idiot owner had simply nailed raw 2x4s to the concrete underneath. Untreated wood in proximity to concrete rots. The sill pretty much just crumbled to dust, and had let enough moisture in that the brickwork was pretty much destroyed anyway.

Unfortunately, it's been that way for a while, and that means there is more damage underneath. I partially exposed the floor joists in the breezeway, and the ends of those are rotted too, though not to the degree of the (former) sill.

For now, the whole thing is just going to be covered up. It's not going to collapse over the winter. It does answer the question of how I can access the underside of the floor, since I'm going to have to do it anyway.