I can't imagine what it could be, unless a insulation blanket came loose somehow. these things have loosened before, but never came off.
Secondly, the thruster would not jar or shake the shuttle enough to knock something loose. I'm thinking they are not saying much right now for obvious reasons. I hope ABC got it wrong but this sounds very serious.
Just my first impressions.
However, if something was already loose, a thruster firing would cause the Shuttle to float away from it. Hopefully it was something small and in the bay. Something from the outside of the Shuttle would be bad news.
On the plus side, they can probably do a couple of small burns to get themselves closer to this thing, if necessary.