What none of Marcus's subordinates or Marcus himself noticed, was that Milo was half dead when he returned, he wasn't even conscious anymore, there was a thin layer of a different "darkness" among the black mist which had fully infiltrated his body, which was keeping everything together.
And that layer of darkness disappeared the moment Milo was in Marcus's presence, it was then when his body crumbled apart due to the highly corrosive effect of the Nethereal's black mist.
Why couldn't anyone in a room filled with high stage Stellar ranks and a system user notice that thing layer of different darkness notice anything was wrong you might wonder, the answer is pretty easy, it's because that was Rita's mana.
One interesting thing Daimon got to know thanks to his tests on Roger, is that the black mist that Nethereals use is called Nether, and to everyone's surprise Rita is completely immune to it.