In our old house, we had (very good) contractors install a bathroom in what used to be a bedroom. For months there were no problems, but now we have an almost-constant odd smell coming from the the shower, possibly from the drain but it's hard to tell.

The floor is a solid-surface Corian-type tray, and the walls are travertine stone. There is a glass door for entry. We had plumbers do a peppermint test to confirm there's nothing wrong with the plumbing, the stone was re-sealed, and a few areas re-grouted. Still, the smell persists. I think it smells like an old sponge (sort of mildewy) but others think it smells more like chemicals. Any ideas? It is driving me crazy.