RDWHAHB...It's just CO2.
I've had the same experience multiple times from both my own carboys, as well as when over eagerly smelling the wonderful smells coming off of the blow off bucket at commercial breweries!
Packaged: Heady Topper Clone (killed during AB's craft bashing SB commercial...), Graff, Basic Spiced Cider
Recent Meads: Mead Day '11 Ginger Metheglin, Mead Day '12 Traditional (orange blossom) Mead, Simple Cyser '12
Primary: Wild Cider, Simple Cyser
Secondary: Why do I keep this line here...?
Bulk Aging: Cocobochet
Planned: Hop Metheglin #3 (NZ hops), Trad. Gesho T'ej