Hmmm - proceeds of gold sale? Price in India is running about $65 per ounce higher than in USA.
That was my first guess as well. Crewmembers smuggling gold into India has been a problem for decades.
Several Middle Eastern airlines had decent BOM and DEL night stops back in the day but most of these have been turned into all night death march turns due in part to the crews often getting caught with contraband by customs.
Another non-drug illicit cash flow I've run across in BOM is proceeds from organ transplants. Often the 'donors' are slum dwellers and the harvesting is very shady. Well-to-do foreigners pay large sums in dollars in cash to get a transplant without the wait back home. Desi doctors in the States get a significant cut of the proceeds for referrals from what an Indian colleague tells me.
As always, some of this smuggling has help from customs with a little
baksheesh given to get the wave through the inspection.
Some recent allegations of an insider gold smuggling ring at BOM airport in this article:
Top Customs officials running a smuggling racket at Mumbai airport? - News