I'm having the same issue. Strangely... it appears to be worse when it's cold outside. I have checked the trap for the shower and it looks fine. At this point I'm thinking the toilet is the culprit. I'm trying the bleach trick now, while I trek out to Lowe's for a plumbing snake. I'll update this post later.

Hi tagilbert and welcome to the Plumbing pageat at AskMeHelpDesk.com. You piggybacked on a 6 year old dead thread so I gave you one of your own. In the future, before you post, look in the upper left hand corner for the date.

strangely... it appears to be worse when it's cold outside. Not so odd! Sewer gas raises with heat and falls to where it can be smelled when it's cold.
Do you hear any bubbles when something's draining? Any more details you can furnish? Back to you, Tom

" I'm trying the bleach trick " what rick is that? This could also be a failed toilet wax ring. Where do you smell the sewer gas most?