Bad smell from bathroom - plumbing
The thing I can't understand is how it's possible if I have a siphon that always keeps a high level of water such that it blocks the passageway of air...


If the building vent gets clogged, then waste falling through the common downpipe can suck water out of your U-bends/P-traps.

The solution is one of 


Get the building supervisor to fix the vents for the common downpipe(s)
Add an air-admittance valve to the plumbing for your apartment.
Replace your traps with anti-siphon traps.


Also, If you leave a sink/bath/shower unused for a month, the trap can dry-out by evaporation. This is mostly solved by pouring a litre of water down the unused sink/bath/shower once every few weeks. I read that some people add a layer of oil (maybe a tiny quantity of vegetable or mineral oil) to slow down evaporation.



Linked examples for illustration only, seek local equivalents.