The first name is Christian, thanks for asking and responding to my post. Let's see, lots of questions. The toilet leak was about a month ago. Both the slate and grout have been sealed with TileLab's Gloss Sealer and Finish. The kitchen is on the second story of a townhouse (everything, including walls, floor and kitchen soffit are made of concrete here) and it's above grade.I don't think rain is getting in. We haven't had much rain here lately. What I think might have happened is that the water from the toilet leaked by the wall and got under the floor. Since the floor is not perfectly level (this place was built in 1942), it settled in the area where the grout was stained. There really are not any moisture problems anywhere else in my place. I was thinking about digging the grout out with my dremmel and regrouting, but that's a lot of trouble if the problem resurfaces. I wonder if it might help to leave it unsealed to allow any moisture to escape. Any opinions?