I have a septic system as I am not within any city or village waste water area.
I have a toilet in my shop that I keep neglecting to repair, but I am pretty sure has a bad seal between toilet and the plumbing system, not bad enough that you ever see any water leakage from under the toilet, but once in a while there is sewer gas in that room. The only other sewer connection in that room is a sink, and the trap is always full so the gas can't be coming through that drain. I think it is change in atmospheric pressure that causes the gas to be pushed into the room. The shop bathroom (the only room in the building with any plumbing) unfortunately doesn't have a vent like it probably should. There isn't much of a siphon problem when flushing toilet though because the run to the septic tank is probably only 20-25 feet max.
My point here is I think the vents on plumbing systems not only are there to prevent siphoning water from traps, but also to equalize pressure in the system when atmospheric pressure changes, so they will "inhale" as well as "exhale" at times.