Bare with me on this one, having read ascendant justice's take on the demo and how the ODST's are protecting the ONI building and DARE is some ONI lady- what if she's there becasue inside that ONI building is something she wants.
If this secret thing is something like technology, weapon or Spartan related that'll be fine but if it's something like forerunner atifacts and flood specimens from perhaps the Spirit of Fire then i'll punch a baby.
* edit *
I realise that the flood have not landed yet because I'm not an idiot and have played Halo 3. But i read that ONI are aware of Forerunner technology (and therfore the flood) before the events of Halo. Is it not conceivable that in some form of Halo Wars cross over they have a flood speciment in storage in that ONI building?
Also ask yourself why Bungie has not confirmed the removal as Flood as enemies when they were so quick to confirm that elites were out?
[Edited on 06.03.2009 4:56 AM PDT]