Pete
Could be many things but most likely drains,
If you have no washing machine or dishwasher then its most likely the little white outlet drain which can be blocked which stops the smell, I used a wine cork pushed in from the back of the fitting.as i dont have a dishwaher.
Also many apartments left unatended smell due to drains drying out in the shallow u bends. (water evaporates quite quickly) leavingsinks etc open to the drain. For this reason many keyholders run the taps and flush the loo on their visits.
Leaving plugs in does not stop this due to overflow holes.
Worst case the seals on the floor drains the ones with silver covers are not sealing or the drain itself is not connected properly, or damaged.