What's going on here? Leaking?

[Mattt555]

The bathroom was already done when we moved in, so not sure what's going on here.
Nothing seems to be leaking but these tiles have discoloured for some reason.
I didnt know they could get wet from the back side of them?
Any ideas gratefully received
Cheers
Matt

20200419_105822

• Mattt555
• 19 Apr 2020

Tile discolouration

 

[footprints]

Just a very long shot but could it be residue from masking tape left on there?

 

[Mattt555]

The markings or colours are beinhd the glaze of the tile so it's nothing that's been put over them

 

[JaimieH]

My guess is that the bottom row is from a different batch of tiles and the difference in shade wasn't noticed prior to fitting.