The last few weeks there has been a nasty mildew smell coming from the master bath. It's not a drain or septic issue as far as I can tell. It's now all be bleached and toilet bleach cleaner down the drain... Still comes back.

I am wondering, is it possible there's water under the tiles? I don't see sign of it but the smell is hidden somewhere.
It's a custom tile job, he built the stall with some orange schluter material and foam (I think) then it's capped in quartzite.

House is 4 years old.