User blog comment:Xaero Dumort/Just announced: Mass Effect 3 : Retaliation/@comment-5008642-20121006022421/@comment-4950455-20121006074256

We know there were multiple Collector ships, so it's possible there were remnants of some kind.

However, I think it's just as likely that the Reapers kept some (metaphorically or literally) "on ice".

Or the Omega DLC might cover it (I doubt it, though).