For starters, I'm not a plumber. (Nor to I want to be one.) If the smell is gassy, then it's most likely sewer gas. W/O seeing the setup, it's hard to tell exactly what's going on. It could be mold/mildew smell, it could be stagnent water that seeps in from time to time. What you might want to consider doing is calling the plumber and asking him to stop by 'when he is in the neighborhood'. Hopefully he's a local plumber. If not, you may have to call one in anyways. There may be a fee because time is $$$.
Could the smell be from spilt beer that ran into the closet and never got cleaned up? You never know. My best to ya and hope this helps.