Thanks guys.
I was not aware of any fumes from the engine bay at all. If it was coming from the drain holes I would expect it inside the cabin as well. I will have a good sniff about the engine bay but doubt anything like that. It is odd as does not happen when the windows are open and would expect that if it is a reverse vortex that it would go through the windows too. Likewise if was ab engine leak.
Dedras used to be terrible for this, I had a turbo and it was diabolical with the roof open. I think it was the drain tubed and was down to different air pressures. The car was even looked at under warranty but not cured. I fixed it by extending the tail pipe out a bit further and adding an extension to the drain pipes so that they sat in the underside airflow. This meant there was no significant air pressure difference between the end of the pipes.
Fiats cure involved sealing of seams in the bodywork.
With the HPE I'd wonder if it is getting in around the hatch seal. Opening the sunroof would create a low pressure area pulling the internal air towards it.