Marks/discolouration on shower tiles?

I have the same issue in both of my showers (same tiles). There are marks/discolouration on several tiles. Some of these tiles are on a wall behind which there is the plumbing, but also appear on tiles where there is no plumbing behind. Most of these tiles are well above ground level. Any ideas for cause / solutions?

Could be a leak behind the tiles coming through? I see a little bit of the grout is missing in the upper left, I would probably not use the shower for a few days and have someone check with a moisture meter or leave the water off (shut off supply and drain the lines) to the shower for a week if possible and see if they disappear, I know you said they appear on tiles that have no plumbing behind but maybe the moisture has spread? it could also be a manufacturing defect of the tiles or overzealous cleaning by someone with possible a corrosive product? I am not sure, I hope you find a resolution.

Thanks for the response. I have left the shower unused for a week, with mains turned off (although did not drain the lines), and no change to the marks.
I have had one tiler have a look, and explained to him my cleaning processes, and products used, and he has no idea! He doesn't think it is moisture coming through, and not a problem with the cleaning products. He has given me the name of another tiler to have a look, and hopefully I can get them in soon.