Thanks for the ideas. I am working on confirming the source of the whiff, which will take a few days. I have closed the roof vent and cleaning the drains more to see if there is any whiff after that, then if not, I will open the roof vent and see if any then to confirm my original suspicion.
The whiff is definitely black not grey when you can smell anything at all. The tank vent on the roof has the usual rain cap. The bottom of the vent at the top of the tank is visible and sealed, so it is not there. None of the grey drains has an air admittance valve, but that may not matter if there is no smell with the roof vent closed. I will know about that later.
It will all get solved after a while. There is no rush, and thanks to all for things to check.
