Unknown stain on ceramic tiles

[Graeme M]

This has got me stumped.

Six months ago, I had a new bathroom done with new white ceramic floor tiles laid on to plyboard which was laid directly on to the joists

Everything was fine until yesterday when I noticed two patches with a very light stain.
Thought it might have just been wet, but it wasnt
Checked all around for leaks - none
The floor always gets dried as soon as we've had a shower

Later on, the patches had spread and again this morning, spread even more - and the shower hadn't been used since yesterday evening, so makes me think it is nothing to do with lying water

It cant be water coming up from the pipes as it would have to penetrate the plyboard, the tile adhesive and then a ceramic tile to do so - I'm sure it would come down through my kitchen ceiling first!!

The only thing I have done is put our bath panel on by glueing a baton to the floor and then screwing the panel on.
Now, I thought that maybe the glue from the baton had emitted some chemical that is oozing in to the tiles as one patch is next to the baton...but the other patch isn't anywhere near the baton

Any suggestions would be welcome

 

[Diyisfun]

If you hadn't said that you had checked for water, that is what I would have said. I had similar problem years ago & it was water creeping along (capillary action) had me stumped for weeks.
Sorry I cant help on this one