Sewer gas coming up through shower drain?

I am getting sewer gas up through my shower drain and only that drain. It is definitely sewer gas and not bacteria from a clog etc.; I know the difference. It drains perfectly also. I suspect that the trap is drying out because it goes away temporarily if I run some water in there. This is a shower that is used regularly, so it is not evaporating from lack of use. The smell comes up even when no water is running elsewhere in the house. The toilet flushes fine in that room and all the sinks throughout the house drain well, but the toilet in the bathroom on the other end of the the house has a huge bloop sound and air bubble when it is flushed. I mention the bloop because I read something vague about that, but it wasn't really clear. This is a 7yr old condo and I live on the upper floor. This is a new problem so it cant be from poor plumbing design.

I'm mentioning all the above so it isn't suggested because I've read that stuff in my research before consulting here. Could it be the roof vent, and if so, why now and not before? I'm praying that you suggest that is the problem, because then it falls on the association to fix and not me. The accessible neighbors in this building are rotten to me, so I can't ask them if they are experiencing the same problem.