Were in the apartment is the smell coming from?
If it is the bathroom then it could well be coming up through the plug holes in the bath, sink and biddet as in spain they donot use any water traps on the plumbing.
We cured our slight problem by running water and bleach through each and then keeping the plug in place when not in use.
If it is coming up this way it may be a problem from the sewers.
I think Jardin 10 had a similar problem earlier this year as when we were out in June PW were digging up part of the bottom of their garden.