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

[David42]

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 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. Although I guess there could be a leak in the attic duct work.
(3) We had the coils checked for mold - none found.
(4) The auxiliary heaters are not coming on so I doubt the smell if from them.  When the heaters come on, "aux heat" is displayed on the thermostat.  We have the thermostat set to "economy" so that the heaters rarely come on. And when they do come on, we know it by the thermostat display.
(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.