Hmmm... I just went through something similar a few weeks ago.

The thing about that type of odor/gas is that it will rise and linger depending on air currents in the room. You may not be smelling it near where the source is.

I will tell you that with our shower, we have been squeegee-ing the walls, and rarely use cleansers in the shower, and I have lots of thick hair that can get stuck in the drain (but not clog it) and what I found was that after I had cleared a big hair clump, the actual drain trap was stinky. I had been smelling it for a week or two- it smelled like sewer gas; I poured 1/2 cup of Clorox into the drain trap, let it sit overnight, and now is 3 weeks and haven't had it again. I think my trap simply got funky. Another time a plumber didn't put the house trap on tightly and one corner of the basement was where the gas collected... 10ft from the source.

So look around, not just where the odor is, cause that kind of odor is rarely sitting right at the source! And in the event you haven't been using that shower, make sure there is water in the trap (which prevents the sewer gas from rising out of the drain).

Thanks Pepperidge Farm! The smell is intermittant, I might not smell it for a few days, then it will be back. I am going to try the clorox trick tonight and hope it's a simple fix. Keep your fingers crossed. :)

In our old house we used to notice that the toilet would stop up some (drain slowly) if the shower drain was getting plugged with hair and soap scum. A little Zep Drain Cleaner down the shower drain and all was well.

I found that I had a large hair clog in our drain. Augered that out and no more smell.

My basement drain smells if I don't pour a bucket of water down it periodically.

How did he vent the new system? He didn't use an air admittance valve anywhere did he? If he did the flap could be unflapped and allowing gasses into the room.

If any of the fixtures are not used on a regular basis, the traps can dry out and admit sewer gasses.

Wax seals under toilets can blow out if the toilet clogs and it's given a vigorous plunging.

My kid's sink and tub get gnarly from hair and soap/shampoo residue collecting in the drains. I first attack them with an old toothbrush, the bristles are great for capturing and puling out gobs of tangled hair. The smell is pretty nasty.

If there's any problem with the house venting system, sometimes you can see toilet water "bubbling" when there's a high-capacity discharge somewhere else in the house. A washing machine, for example.

It's springtime, are your bathroom windows open? Is there a roof vent nearby? Not likely, but occasionally odors an be brought into the house that way.