Anti vac traps and Air admitance valves are all one way vents into a drain to allow air in only (not air out). So if correctly functioning you will not get smells from them.
Howver if either is faulty then it is possible for air to come back into the room where the valves are. The other issue is blowback where the pressue in the drain is greater than normal air pressure and air is forced back through the trap - usually with a gurgle if you happen to be there at the time. This can be caused by incorrectly vented drains (i.e. no vent at the head of the drain) or by blockages (even partial) causing a temporary increase in pressure.
So with that information to hand you may want to investigate further. Certainly worth lifting a local man hole cover to observe the flow of water when a WC is flushed to see if the flow does not seem to simply flow away freely.