During Stereo's showdown Keitel wonders that "Zille" would surely not come alone. He asks Deisler how many men they have down there but when Deisler answers that there are some ten but more on their way, Keitel immediately identifies him as a traitor, shoots him in the head and orders Hansen to inform their men that there are enemies among them.
But I didn't quite understand where Keitel took such a fast deduction from, seeing that he must have been quite sure to just shoot him right away. But maybe I missed something, as Keitel seemed to have taken a look at the surveilance cameras when he talked to him. So how did Keitel deduce that Deisler worked together with "Zille" (or at least with Gaspar)?