Just because you mentioned plumbing ...  check to make sure you don't have a plumbing trap that has evaporated dry. 

I just found one this morning in my shop that had dried just low enough to allow sewer gas to 'make an entrance'.  I was trying to blame the dogs for the awful smell, but upon closer inspection, I found the drain trap was the source and solved the problem with a couple cups of RV antifreeze.