It only really just occured to me about the Alpha Legionaires final words in The Face of Treachery. "In times such as these, even the most trusted face can conceal an enemy."
Now, whilst quite easily just telling the World Eater that spies can be anywhere, what is the point in telling him? It's almost a 'no shit Sherlock' style statement to make to a dying World Eater, but more meaning to us perhaps. Now possiibly and probably a long shot and a leap, but Valerius spent a good deal of time trying to gain Branne and the Raven Gauards complete trust as is finally rewarded with it. And then only in the next scene we are shown what trust can lead to. Again, bit of a leap, but interesting to me nonetheless