Hard water and the grout lines
Beverly Hills water leaves a white crust on shower glass and in grout joints, and the city's own figures for how hard it is don't agree. The Water Services page gives an average of about 17 grains per gallon. The 2023 water quality report puts it at 4.7 to 8.9. Part of the supply comes from the city's own wells and the rest is bought from the Metropolitan Water District, and the city says drought and the way water is distributed move the number from year to year.
Either way, the minerals end up in the grout. Sealed grout wipes clean. Grout that was never sealed soaks up the film, goes dark and starts to powder, usually at the corners of the shower first. A cracked tile lets the same water reach the thinset behind it, so we tap the tiles around it and pull any that sound hollow before we regrout.