Probably,
Spark, in the end of Halo 3, before the firing of Instalation 04b, said:
343 Guilty Spark: (Shocked, Alarmed) "Bu-bu-but a premature firing will destroy the Ark!"
Johnson: (dismissively) "Deal with it."
343 Guilty Spark: (quietly) "...will destroy this Installation."
And, as the dreaadnought must have landed on the ark, ans has Truth died, I doubt anyone remebered to pick it up.
Rafter
yeah good point but i really don't think covie stragglers would "forget" the most powerful ship in their fleet