Sewer gas smell in unfinished basement

We just moved into a new house with a septic system. The house was built with an unfinished basement but roughed in for a future bathroom and sink.

After being in the house 6 weeks we now notice a sewer gas smell in the basement. However, all the rough in plumbing was glued and capped. There is a floor drain and a utility sink that both go to footer drains. There is a shower drain and toilet drain on the other side of basement which is where the smell is, but both capped

Any suggestions on how to troubleshoot this? I did call the builder but would prefer to see if I can figure it out before he comes. He is Amish and not local so he may not be there today

TIA