Diagnosing a smell from air vents

[BocaBill]

Seat ibiza Bocanegra
1.4 Twincharger

So for a while I've had this sometimes sweet, sometime oily smell coming from my cabin air vents, the smell ONLY appears briefly once upto temp and when I've come to a stop (traffic lights, stop start traffic etc, when on the move it's just clean fresh air.
I had the AC pressure and leak tested earlier this year and fixed refrigerant leaks that were found however smell still present.
No loss of coolant, hot air works fine, no creamy engine oil, no fluctuations in hot, cold air, engine coolant temps are spot on.
Putting on the AC eliminates the smell and I've noticed on occasion my AC compressor covered in a black oily substance however not losing refrigerant pressure since leaks were fixed..

I've also replaced PCV breather, PCV Breather Valve & Evap valve.

Smell still remains....

Any thoughts on what, where else to look at?

 

[Yern]

Could it smell like hot engine coolant (maybe take a sample and smell when hot ). I once was servicing a car and smelling this faint smell in the engine bay. Pressure test no leaks but on closer inspection found staining around the radiator inlet pipe stub. Sent the radiator away for repair suspected slight weep when cold which sealed when hot, cured it. Maybe heater rad problem, a 'leak' would normally mist the front window?

 

[BocaBill]

Could it smell like hot engine coolant (maybe take a sample and smell when hot ). I once was servicing a car and smelling this faint smell in the engine bay. Pressure test no leaks but on closer inspection found staining around the radiator inlet pipe stub. Sent the radiator away for repair suspected slight weep when cold which sealed when hot, cured it. Maybe heater rad problem, a 'leak' would normally mist the front window?

I'll take a closer look of the rad, got no misting windows or wet footwells