well, he is/was a guardsman, so where all the men following him. It would be -really- easy to just say "yeah, we found this squad of guard and had them follow us with the marines just in case" and that facade should hold up to cursory scrutiny while everyone is distracted by the dead commissar and suddenly Dark Angels. 

or atleast long enough for Oskar to blast a good portion of the command crew with flaming promethium.