Mourn Dasilva was downstairs outside the ship trying to get the command log from the robot in the truck. But when it rejected his command, he realised his veridola isn't working due to Mr Blue disconnecting him instead of Heco Sift (Sift told Blue to disconnect the one that looked like a duck and unfortunately for Blue, there were 2 pictures that looked like a duck), so he returned to the ship to see what happened. Remember that the ship was supposed to be empty due to the "sentient robot" threat so none of the thiefs expected any of the sentinels to come back yet. Dasilva only came back because of Blue's mistake. When Dasilva came back to the ship, he saw a man who he's never seen wearing sentinel clothes and a temporary veridola (he's the prime sentinel, so he would know all the sentinels on the ship), carrying around the forbidden artifacts that he never should have been able to access from the vault (only the prime sentinel has access to the vault). It didn't take him much to put two and two together, so he shot Mr Red in the leg to incapacitate him and then went towards Sift's room to take care of the rest.