What IS That Smell ?!?!

For the past few weeks, there has been a strange and unpleasant odor in one spot of our home. It occurs on and off with no obvious pattern. It is near a bathroom, but is not coming from the sink or toilet. The shower drain could be the source but I can't be sure. Last time I smelled it I thought it was the drain, so I used baking soda and vinegar to freshen it which seemed to work. But now it is back. That shower is never used since it is on the main (non-bedroom) level of the house. We do have a septic system, but it seems to be working just fine and besides, when that has backed up in the past it was a different (worse) smell and came up through the drain in our basement. This smell is just in that one spot on the main level.
The curious thing is that sometimes the smell seems to be coming from the air intake vent right next to the bathroom that smells. Do you think maybe something died in the wall and I'm just getting the occasional whiff of decomposing rodent? There are mice all around us outside in the woods but our two cats keep them out of the house (or at least we've never seen signs of them in here). Any ideas? It needs to go away soon, I'm getting more and more grossed out just sitting in the next room from it. YUCKO!