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The D
I must admit I hadn't thought that water could be coming from the walls...but if it was wouldn't we be seeing damp patches on the wall grout, too? These have not been replaced with epoxy, but dry out very quickly
I must admit on the one job there was signs on the wall but on the other there was no sign on the walls at all the water was just traveling through the adhesive and gathering at the lowest point. yours may not be the same problem but to be honest IMO grout will not let that amount of water pass through in a domestic shower without some serious cracks or pin holes. try cutting in to the grout on the dark patches and see if there is water trapped in that area.