I have always seen that very thin layer on beer that has completed fermentation and is has cleared...in both glass and plastic. I have always attributed it to the very small quantity of organic oils present in beer that make their way to the surface once everything has settled down after fermentation.
Something is always fermenting....
"It's Bahl Hornin'"
Kegged: Sour Saison, Pale Ale, Aggie Ale, Firestone DBA, De Koninck Blonde
Bottled: Belgian Quad (Grand Reserve), Derangement (Belgian Dark Strong)
On Deck: Pliny the Younger