Passat interior staining

Got a 2000 Passat with some odd black speckled staining to the interior (beige cloth) mainly on the back shelf that looks like it has been caused by an a/c ventilation leak or something. Tests show it is not exhaust or oil based, trying to find cause.

VW perplexed as everyone else is and not being that co-operative

Anyone else experienced this problem with their Passats?