HVAC intermittent smell from ceiling vents - How to fix??

[GoshenPlumber]

Heat pump American Standard
Compressor 3 ton model 4A6H4036D1000AA
Air Handler model TEM4AOC36S41SAA
APCO UV-C system mounted in duct upstream of air handler coil on 24 hours/day

Heat pump in condo in SW Virginia. Compressor is on the building roof, air handler in utility closet in hallway. There is a very intermittent (once every 2-3 weeks or so) smell from the ceiling vents that only happens maybe once every 2 weeks or so, and is only smelled in the winter when heating. I have never noticed the smell in the summer when cooling. To me the smell is somewhat 'sour'.
(1) We tested and this is not a refrigerant leak.
(2) Not from outside air since our duct work does not bring in any outside air.
(3) We had the coils checked for mold - none found.
(4) I don't think the auxiliary heaters are not coming on so I doubt the smell if from them.
(5) The intermittent smell started before the APCO UV-C system was installed.

QUESTIONS:
(A) Could the smell be from a coating on the coils?
(B) Could the smell be from a coating on the aux heaters even when the heaters are not on?
(C) Could the smell be from a leak / hole in the attic duct work?
(D) What else can you suggest?

Thank you.

 

[Fitter30]

Condensate drain from the ac is it piped into a stack or sink after the trap?
Floor drain dried up.