Did the new septic system include new piping inside the house? Did any of this new piping happen in the same room or area that the heatpump air handler (indoor fan unit) is located? Are there registers in this area, especially return registers?
Possible things that might be happening are: If new piping or any piping was done inside the home it's possible that an existing joint or fitting was moved or loosened so that it leaks sewer gas. Maybe when they did new piping they put in a fitting that was supposed to have a pipe connected to it, but somehow it didn't get connected. You might be closing doors to the room where the sewer piping and air handler are located that you hadn't closed before, this could (if a return register is located in the room) pull sewer gas even through a drain that has the proper amount of water in it.
I spent 3 days trying to trace down this very thing once. Finally found a new pipe that had been installed that somehow had a saw cut on the top 1/3 of it. It was buried on the ceiling of the basement tight up against the floor above.
You can call your heating person and ask them if they have a gas leak detector. Sewer gas is methane and the detector can track down the source if the person using it is skilled and thinks a little outside of the box.
Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.