In a brand new construction 2 story home with on a new Septic System, there is a foul odor detected in the Master Bathroom private Water Closet room which has just a Toilet and is located on the first floor. There is a Full Bathroom on the Upper Level, not directly above this toilet, which has no issues and a Powder Room Toilet on the same 1st floor level which also has no issues. The owner has noticed the smell from the day of move in almost 5 months ago and it is like a sewer, musty, just gross smell despite constantly cleaning, at times it emanates through entire Master Suite. No evidence of leaks, nothing apparent in crawl space. Plumber/HVAC installer of new home venting claims all is vented properly. There is one single vent pipe protruding through metal roof which appears to be 2 inch and roofer has checked twice that it is not covered.
What could be the cause, and what could be the solution?
Here’s my WAG… With a single vent it is very likely there are air admittance valves installed under the sinks, I look there first.
I agree with @jburkeson1
No matter who says what, that home is venting sewer gases to the interior. That builder owes a fix, even if he has to start tearing out the walls until the open pipe is located. If he is lucky, it is just a faulty air admittance valve.
If the traps are filled there is a broken pipe or a missing cleanout plug.
is there a floor register in that room?..did You look inside ?
One very common problem is the drying out of a trap in a basement floor drain or unused toilets, sinks, and showers.
A wax ring that does not make contact with the bottom of the toilet will not leak but will allow sewer gas to enter the room. This sometimes happens when a short wax ring is used for a water closet that is installed on a tile floor. It only takes a small void in that wax ring to allow the gas to escape. I would recommend pulling the toilet and resetting with a double ring or a tall wax ring with horn.
Makes a lot on sense in that the only plumbing fixture in the water closet is the toilet. Start with the obvious and work your way from there. A lot cheaper than tearing out walls at this point.
Plumbers often use a single wax ring for a toilet that is installed on tile. You need to use a double ring or a tall wax ring. 
it’s really the only thing that makes sense.
Thank You for that advice, hopefully it’s that simple.
Yes, Thank You, it needs to be corrected no matter what extent it takes.
Yes, there is a floor register in this small room, and it has been checked, no apparent issues seen in that register.
Thank You for that tip too. The new home is on a crawl space and the other toilets, sinks and showers are all used on a pretty regular basis. So, are you saying to advise the homeowners that all the plumbing fixtures should be run occasionally to keep water in all the traps throughout the home?
Thank You, that sounds like a good possibility of what might be going on here and would definitely be an easy fix for the plumbing /HVAC contractor to correct before tearing out walls!
Yes, since we didn’t see firsthand what exactly they did at installation, this could be the case and makes sense. Much appreciation for the responses/advice here!