The gas cloud could apparently be seen from the ground. Someone snapped a pic from Reunion island, near Madagascar: http://wikkit.tumblr.com/post/62684205892/tracking-a-new-spa...

EDIT: Here's more pics from South Africa http://uforsa.co.za/2013/09/mass-ufo-sighting-on-29-septembe...

Makes sense. I can't really think of what could've exploded on the second-stage without also causing the entire vehicle to be obliterated. Whereas a bunch of globs of kerosene fuel would promptly freeze and be highly reflective.