That looks like a water leak behind the wall. Thinset drying out would be in random splotches not a long downhill pattern like that. It just wouldn't be that regular and it wouldn't be darker near the floor. Water spreads when it hits something and runs downhill, so that pattern is consistent with an active leak. You said "the leak happened when we turned the water on..." Of course it did. If the water was shut off to those pipes and fittings, what would there have been to leak? If I can tell that the wall is wetter near the floor, that's not just a little dampness that you can dry out with a fan or a hairdryer. The best advise here is to turn off the water if you haven't already and remove drywall on the opposite side of the wall. That is probably the only chance you have to save the tile if you still can. Bigger potential problem is that if you don't, the area is likely to stay wet enough long enough to create mold and mildew problems -- even rotting or draw termites. The longer that sits wet or gets wetter, the bigger the problem you will have in cleaning it up and fixing it. I hope we didn't hear from you today because you've been busy doing that. Good luck. This post was edited by lascatx on Tue, Mar 19, 13 at 0:14

Honed will be less obvious. If it's etching, sealer or honing won't prevent it. I'd question whether it's just water, and what is in whatever you're mopping it with, too. (I only clean the marble with marble cleaner.) My only spots are around the toilet... Nothing to prevent that short of sitting. This post was edited by KevinMP on Wed, Apr 24, 13 at 23:04

I'd prefer that more grout had been left in the joints. They look dished out a bit too much, where too much grout was removed during the cleanup phase. I'd have preferred the grout to be more flush with the face of the tile. Looking closely at the first two photos, the darkened grout lines are all parallel to one another. In the first photo you can see the wet grout lines are all parallel to the edge of the drain. That leads me to believe that the "uphill" edges of the tiles are acting like mini dams and holding water, they are preventing water from easily flowing downhill to the drain. The second closeup photo, you see the same effect. All the damp grout lines are again parallel to one another. Again, to me that reflects a drainage issue. Not necessarily with the amount of floor pitch, but with the grout lines being so dished out that they hold water and slow drainage. Marble is porous and can absorb water. With the grout lines being a bit too raked out, the edges of the marble hexes are exposed. Water held against the tile edge can lead to absorption into the marble. The crack in the grout in that second photo, you also wrote that there are other cracks. That's usually a sign of too much water used in the grout mix, or that sanded grout should have been used instead of unsanded. Unsanded should be used in joints less than 1/8" wide. Cracks can also indicate movement in the floor, but that shouldn't be an issue with a properly set Kerdi Tray. As far as him having used a preformed Kerdi-Tray, if he did, it appears that is was certainly cut down in an awkward manner. The drain is no where near centered in the shower. So if he did some filling here and there with mud or thinset to even out the floor pitch or edge elevations, that could have be part of the problem. But that's supposition on my part. About the only other thing I could question is that often times with small tiles, when the sheets are set into the thinset, thinset can ooze up between the hex tiles and partially fill the grout lines. That thinset needs to be cleaned out so the grout lines can be completely filled with grout. If it's not, you can get uneven thicknesses of grout, think of it as a veneer of grout over a blob of thinset. That can result in uneven water absorption, the grout cracking, etc. All that aside...with this being your second go-around, he should have install the sloped Kerdi Tray and thinsetted it to the subfloor. Then he should have set the Kerdi drain. Then he should have thinsetted Kerdi Membrane (an orange colored sheet material) over the Kerdi Tray, then tiled on the Kerdi Membrane. I'd be interested in how he tied the Kerdi Membrane in with the wall membrane IF he used Kerdi Membrane over the Kerdi Tray. Best. Mongo

I've never seen them appear the day after, but that doesn't mean it can't happen. Although I can't say for sure, it's usually lower grade carrara that this happens to, but it DOES happen to carrara.