Where is the smell coming from?

I work in a hair salon on ground floor above is a beauty spa, every now and again we get this really strong sulpher smell coming from the bathroom area (it really is sulpher) we had the firemen out to check for a gas leak they couldn't find anything not the spa upstairs smells it also they have showers up there that are in the same area as our bathroom above (they say perm lotion) can't be because sometimes we won't do a perm for weeks and we still get the smell. any ideas what it could be and where coming from???