I would assume Cloaking tech, I was wondering that myself though tbh....stealth corvettesthe answer is penisdamaged the relay so they could not spot them.... you have done winter contingency right?
The guy who asked this has a point... in all the books except Contact Harvest there are signs of them ("whispers near Io" Earth Halo 2) and there was another instance in Halo: The Fall of Reach (book sigma octanus (spelling i know!!))
what this guy is asking is how did they get planetside without ANY knowlege by the UNSC?
In my opinion, although Reach is a great game it could be better - read the book, the Fall of Reach should have been EPIC!!! but instead they created this whole other part of the conflict and kinda ruined it.