You need to find the source. By trial and error, I would start ruling things out. Pour some water down the shower drain and see if you get the odor. It could be as simple as muck lining the drain pipe. If you are so lucky that it is, pull the drain cover and give the pipe walls a good scrubbing. Put a dab of bleach in there, too. How was the shower built? I've seen bathrooms where drywall was tiled, and when it gets wet, the smell is funky. If you have tile, cover it with painter's plastic, take a shower, and see if you get the odor. See what I mean? Start isolating parts of the bathroom and rule things out.

You must understand that you are asking us to resolve a problem sight unseen and with very limited information to work from even though you have already had a plumber on site and he/she was unable to find the problem. In my previous post I was not making any assumptions, but rather I was pointing out a potential source of your problem based upon current code approved construction methods. Having now ruled out attic vent termination we must then turn to other potential, although seldom noticed sources. The four principal gasses that make up sewer gas are Hydrogen-sulfide, Sulphur-dioxide, methane and carbon dioxide. Of these four gasses carbon dioxide is odorless and while hydrogen-sulfide has a very distinct odor, it also causes an anesthitation of the olefactory nerve in rather low doses. This means that while hydrogen-sulfide is readily noticeable at very low concentrations, as the level of concentration increases it will shut down your ability to smell it rather quickly. It must also be noted that hyrdogen-sulfide and methane are heavier than air gasses so they tend to stratify at low levels in any given space. This means that it could be in a rather high concentration at the floor level in your basement, yet you will not smell it when standing erect in the room. The problem here is that most HVAC units have an updraft air flow, which means the air intake is rather close to the floor where the sewer gasses would concentrate. Once the gasses are pulled into the HVAC unit they will then be distributed throughout the structure by the normal HVAC air flow. You also stated that this structure is only 5 years old so it would be safe to assume that it was constructed with a very tight envelope and knowing state of the art construction it is highly unlikely that there is a source of outside air into the basement. Under these conditions when an HVAC unit is running they will often cause a slight drop in normal atmospheric pressure in the basement, especially if the return air ducting is sized critically or if any opening to the return air duct is even partially obstructed by furnishings or personal belongings. (one prime indictation of a reduction in air pressure in the basement when the unit is running is a basement door that is left open but slams shut when the HVAC unit runs). We must also keep in mind that normal atmospheric air pressure is 14.96psi at sea level and nature abhors a vacuum. Any reduction in atmospheric pressure, no matter how slight, will immediately cause air to infiltrate in from any possible source. Further understanding that all sewer and drain lines are vented to atmosphere it would then stand that if the air pressure in the basement drops even slightly air might be infiltrating into the basement through floor drains, laundry standpipes or utility sink drains. The rate of infiltration through these drains may not be enough to even effect the water levels in the traps, other than to cause them to dry out quicker than usual. One way to test this problem is to keep the basement door open a couple days and see if the problem still persists. If not then the permanent solution would be to install a small makeup air intake into the basement.

The house was built in 1967 and hasn’t seen any major plumbing changes while I’ve lived here. So the house is mostly copper and cast iron. We had a similar situation in our previous house which was built in the 1950s and had copper pipes. I'd been telling my husband for years that I could smell something like sewer in the house and he thought I was crazy. Then a few years ago we had a plumbing problem with the kitchen sink (which shares a wall with the bathroom) which meant opening the wall and found that the copper pipes had corroded quite badly. I suspect you may have the same situation, and the new septic system installation may just be coincidental.

FWIW, our Master Bathroom would get the sewer gas odor from time to time. Scary stuff. During our recent bathroom remodel, I asked our contractor to have the plumbers take a look at it. I mentioned there was a pattern to the odor, and over the years we noticed it when the wind blew from the north. What the plumber determined was that the sewer gas pipe came out of the roof a few feet (to the north ) from the attic ventilation turbine whirly thing. Their theory was that the gas was getting blown into the attic vent and came down thru the bathroom exhaust fan. IDK if that was a correct theory, but they moved the sewer gas pipe far away from the turbine vent and we will see, this winter, if that fixed it, My money is on it being solved. While this may not be what is going on in your house, it could be something odd like ours was. Good luck. I hope you can get it solved!