So a few days after my last post on the 4th, I had a plumber come over and look on the 12th. We had not used the shower since the 7th so I was expecting it to stink like mad when we fired up the shower.

Nope... the shower ran fine with no stink... I was in showering for over 10 minutes. Now, to be fair, we had just cleaned the shower top to bottom the day before.

He went up in the attic and cut the vent pipe... no smell and he snaked the vent each way... nothing was clogged. We ran hot and cold water down the vent pipe for about 10 minutes... no problems. We ran water in the shower and vents for probably a total of 30 minutes which definitely should have reproduced the problem if there was a dead animal in the wall or something like that. We tried all sorts of things for 1.5 hours and could never reproduce even a hint of the smell.

Now, Nov 18, the shower is beginning to stink again!!

I'm at a loss... there's no way 2 people showering once a day each could possibly leave/create that much biofilm on the shower itself or down the drain. Also, it's next to impossible to find out the source of the smell. If there was a dead animal in the wall, then why did it not smell the past 6+ days? If a pipe was broken somewhere or a vent was clogged, why would the smell not be here for the past 6+ days?

Any other suggestions?

Eric,
There is many things that can cause odors to emit from drains. The #1 cause that I have noticed is from soap scum, and the bio-film which lives in this organic matter.

You can snake and use bleach as well as other harmful chemicals to get rid of this problem for a little wile, or you can use natural enzymes and microbial cleaners that will actually continue to feed on this muck that forms in the drains.

I myself prefer to use non toxic earth friendly products.

Sani-Drain makes a wonderful product that suspends from the drain cover or sits in your sink
That slowly leaches out enzymes and microbes that feed on this complex organic matter which in turn dissolves it into water and carbon dioxide. These microbes actually breed in your drain lines doubling every 15 minutes as long as there is muck in your drain to feed on.

A lot of Plumbers don't like this type of product because it eliminates service calls and can reduce waste management by 50%.

Here is the website Sani-Drain, Visually effective Microbial drain deodorizer