When NOT in use, both shower and bath sink drains smell of sewage. Smell goes away with running water. New construction in an addition put on house in 2009. Noticing that the smell is strongest in the morning hours after hours of non-use. What to do?

For whatever reason, it sounds like a vent issue and the traps are being pulled dry. That's why water solves the problem, at least temporarily. Could be a plugged roof vent, revent, AAV etc. Could also be a fixture placement issue, with the toilet drain pulling the traps dry if the vent is placed wrong.

I agree, sounds like a clogged vent and that allow siphoning of the trap. Fill the sink to the brim and the lift the stopper, do you hear and unusual sounds like gurgling?