We used to have a contract with a water cooler company to go and clean up leaks and spills from their coolers in commercial premises and hotels and seemed like the easiest job in the world
Until we realised hardly any of the stains would come out, had to visit one place 7 times once as the stain just kept coming back, they always seem to disappear with cleaning but are back again in a few days.
I tried hwe t/m and portable, l/m, dry granules and getting on my knees and scrubbing even dry cleaning solvent and nothing
I did some research at the time and it was suggested it was residue from the backing of tiles or the adhesive that was causing the stain which was obvioulsy mobile when it was flooded then dried back on the tile.
whether this is the case or not im not sure, but i do know they are a devil to do anything with and it makes it harder as the customer will normally think its just a water mark that should come out easily.
I steer well clear of these now ( and water cooler companies ) but if i have to will tell them straight its probably impossible to remove.
Saying that some are sometimes just water marks
Steve