>He seems unresponsive to words, but a gentle prod from your (de-activated, of course) chainsword seems to make him scoot along
Suspicious, much? Discreetly have 2 of your men watch him, with orders to dispense the Emperor's Mercy/Justice if needed (remember, you can't relax when dealing with daemons). Check out the tanks exterior. If it seems ok, have 2 men on overwatch, 2 men watching the mute guardsman, and take the last with you to check the interior of the tank, ie you watch from the hatch, Freytag goes in.
Do we have a name for our sarge, commisar, lieutenant? I vote Velius Dues.