From foxnews.com: "The object, which is circling the Earth in the same orbit as the shuttle and is of undetermined size, probably came out of the shuttle's cargo bay because some jets had just been fired, shaking the orbiter, NASA spokesman Doug Peterson said."
Could be bad info...
If it's something out of the cargo bay, I'm not sure why the mystery. An object of any size, missing, would be obvious, I'd think, and not too difficult to visually inspect.
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.