We have the same problem and at one point moved out of adjacent bedroom.
The cure was to gain access to the empty apartment and flush the sinks, bidet and toilet as they do completly dry out which makes the apartment part of the sewage system, and it is not pleasent, the smells do transfer into the cavity walls and the sockets are the main entry. Another problem is the washing machine and Dishwasher outlets in an empty apartment these are open to the sewers.
Frequent access and flushing, and sealing of the outlets helps.
If you can not trace or contact the owner, ask Administrator to do so saying if it is not dealt with by the owner you will pursue with the Town Hall Health Department.
In our case we can get access but have to chase them every now and again.
Good Luck