If its an intermittent problem. It could be from standing water that gets behind a wall at certain times of the year.
If you get standing water behind a wall, after a week it will smell like sewage and rotten garbage.
Check that someone isn't splashing water from the shower into a crack on occasion, or during certain times of the year when the temperature is just right some water may be seeping into the house from roof or gutters?
Just a thought to check out...
Let us know when you find out what it is. Its usually a good lesson learned to share with others.