I've been answering questions about outgassing for years but your title caught my eye
I checked around with colleagues and we came to the same conclusion, as the other posts, that its most likely mildew/fungus/mold caused by the water/sponge in a sealed environment and/or possibly outgassing from the plastic bag itself.
Epson makes a cover for the 3880 which is designed to keep dust off of the printer.
Because it fits over the top, air can circulate underneath and there has never been a report of what you are experiencing.
And of course any cover will work to prevent dust getting into the printer.
Dan (Dano) Steinhardt
Marketing Manager, Professional Imaging
Epson America, Inc.