I posted on 1/31 that I was having a periodic sewer gas smell in the basement of our less than 1 year old home and got some good responses. I thought I would post what ended up being wrong.
There was a 3" vent pipe for the 1/2 bath that was open to the basement. A smoke test showed the leak almost immediately. The vent was hidden behind the heating ducts. The plumbers capped it off. No more bad smell.
Thanks to all who replied,
Ken