I was wondering if this is due to our foundation cracking. One way or the other, it's due to movement in the structure not being isolated enough from the tile floor. If your foundation is cracking, I'd say your tile floor is the least of your worries. But yes, it could cause the tile and/ or grout to crack. It could also be one of several more subtle causes, such as not putting any thinset under the backerboard when installing it, using the wrong thinset, etc., etc., etc.. Is this a sign of more cracks to come? Should this be repaired and if so how does one repair this? It may or may not be. It would take a physical inspection to determine for certain exactly what's causing the movement. That would also be required to repair it, because before you repair the crack you need to determine what's causing it, or else it'll just keep coming back. Our contractor added another layer of grout. Did the second layer of grout permanently bond with the first layer? That depends. How long after the original installation did he add the second layer of grout, and approximately how deep were the joints prior to the second layer?

"One shower wall is an exterior wall, another has no cabinetry on the other side, and another has very minimal cabinetry." Well then, there you go. It's not due to work that was done on the other side of the wall. ie, no one tiled a wall then had other workers beat the heck out of the other side of the wall, causing the tile to crack. Could it be settling? Sure, But if it is, it's unusual. When a house settles, cracking usually first occurs in the changes of plane (corners) because those are the weakest points in a structure. If the grout between the field tiles within a wall is cracking due to the house settling, that's significant in terms of it being a significant structural issue. Also, you have marble tiles which are not very strong. If you had significant structural issues, your tiles could crack too. But again, looking at the grout itself: pinholes in the grout, raked out grout lines, inconsistent surface texture of the grout within the grout lines. And the cracks that look more like shrinkage cracks instead of structural cracks? Those are all signs of a poor installation by the person that installed the grout. "How can you tell the grout wasn't mixed well? " Because everything you described, and everything I see in the photo, is a symptom of that. I'm not saying I'm right. I'm telling you that's my opinion based upon the evidence you have provided. And that's assuming (rut row) that he properly installed and prepared the tile backer board. If he didn't properly detail the backer board then the seams in the backer board could telegraph through as cracks in the grout. But you provide no information on that. Just the photo. But based upon the information you have provided, in my opinion it's simply a grout installation issue, and it's the fault of the installer. But again, that's simply my opinion. Good luck with your remediation.

Thanks all for your response..... SJ McCarthy - You are correct. Vinyl flooring was previously installed on 1/2" plywood. Sophie Wheeler - Grout was premixed. Yes to thinset between plywood and cement board. We discussed the subfloor with tile contractor and GC; both assured me there was no problem with deflection. I was very concerned about that because the tiles are large format/rectified (also very expensive) and we didn't want problems down the road. I wouldn't say the job was rushed. millworkman - I ok'd the 1/4'' cement board (no, I'm not a pro) because of height. The tiles were pretty thick and I didn't want a large transition in the doorway. Total coverage was only 55" x 57". The grout joints are 1/8" so the cracks are only visible if you're a couple of inches from the floor. I wanted to do a little scrub down of the grout when I noticed them. Should I just keep an eye on it for now? The bathroom is rarely used. Should I have the hubs go under the house, check the joist spacing and see if the substrate was suitably prepped?

3 most common issues are A. CTEC explained it well. Bravo sir. B. A foam shower base with inadequate density spec was used. True story schluter has reworked their shower bases for just this reason. The old version had no spec for tiles smaller than 2x2 and even then they had issues. many foam pans still dont meet proper density for penny tiles and point loads. C. RARE but a pan even concrete built on say a slab on grade where the slab itself has Out of plane crack/movement. VERY rare theres also the rare chance a wood build structure has an structural issue.. Most likely installer error especially if A No show ! but who knows Hire wisely. Hire ONCE