I remember doing a "walkround" [Pre Flight] at Malaga {jump seat, friend of Captain, pre 9/11} and there was this 30 foot plus black , fading to medium brown, stain coming from the air conditioning exhausts {A320}. It was nicotine...... If I hadn't stopped smoking, then that would have been a certain start......
bb
Would not have been nicotine (it's actually tar, anyway) from pack outlets; cabin air does not exit from there. Outflow valves and other leaks from the pressurised area best source of tar in the old days; few, if any airlines allow smoking these days (or are there some?).