(This worked on the test forum...) Here are two photos of the floor--one after it was first installed, the second is current.

In the kitchen forum there was someone who had a stain around their faucet and it turned out that the plumber had used plumbers putty when installing which is a no-no. I'm no expert and I'm not sure it would affect tiles the same way as a slab of Carrara but maybe that's what happened? I understand that you are supposed to use Clear Silicon to seal on marble as putty is oil based.

Also maybe the tiles weren't sealed properly?

There was also a thread on the kitchens forum about poultices to remove stains. . .

I'm no expert, but I'd be willing to bet that there's some kind of leak around the drain that's allowing moisture to wick into the substrate under the tile. It might feel and look dry, but only damp could cause the tile to take on that sort of look and be confined to just around the drain.

It's certainly not an issue with the tile, or it would appear across the entire floor, or at least randomly.

Something isn't right, and I'd want to get to the bottom of it -- which will probably entail taking up the tile.

Actually, just the opposite of a leak. Good chance the weepholes are plugged, and the pan is filling up with water.

Did you get this fixed? We installed for a client and have the same issue. Or similar. The tile guy is experienced. Here’s what it looks like.