MB wrote:
This is just a thought-- probably has nothing to do with anything: I've
noticed in our bathroom that the grout between the floor's ceramic tiles,
particularly near the outside wall, darken when there's lots of moisture in
the room, like when I've hung up something damp, like two bedsheets. I've
assumed that it's condensation on the cool floor.


If it varies with internal humidity, probably yes.

You get a nice thermal gradient across a concrete floor towards the
house exterior.

More heating and ventilation needed?