I have personally poured hundreds of tile shower pans, and never had a single warranty issue. The hot mop method is also a new one to me, although I see how it could be reliable.
I always used the rubber liner, with integrated pan liner drain.
Here are my thoughts:
- Did they do a preslope under the hot mop? (should be 1/4"/ft.)
- How thick did they pour the final shower pan? Also needs to be at least 1/4" per ft slope. Or did they apply tile right to the hot mop?
- Most of the new grouts claim to have a sealer added into the powder, but I do not trust them one bit, unless it was a pre-mixed grout (ready to use). These are better, but only aid in stain resistance, not a moisture barrier like Martin said. A topical would make the water bead on the top, but you need to re-apply every couple years.
- The hollow spot may be nothing more than the sound of the shingles not being fastened to the subfloor. May not be a problem, but if there is physical movement, this can cause the mud pan and the tile to eventually work loose or crack.