The Spanish system of drainage is rather different to that in the UK (and I think this is a Spanish-wide thing, but could be wrong).
There is no separation of drainage pipes - everything ends up in the same place. Smells from drains can be a problem in these circumstances...
This tends to lead to problems when a property is left empty for some time - especially if the water has been turned off.
When we were looking at property to buy, we were actually advised by our gestor (who went with us to translate), that it can take some weeks at least for smells to clear after the water has been turned back on, so we avoided already smelly places.
Like Jo, we sometimes get 'drainy' smells - especially from the pipe nearest to the end of the row (and therefore nearest to the point where the flow joins the main system). It doesn't happen very often and, like Jo again, a flush of the loo and running the taps (and putting water with washing up liquid in it down the sink) usually does the trick.
That said, I would get a plumber to take a look and see if there is a blockage somewhere and then pass this information on to the Landlord. Hopefully he'll then get it sorted out.