It was not possible to avoid guards in the Room of the Ancient Warriors.  When a guard was there, he would always be behind the suit of armor that opened the door.  If this was not the case, theoretically, it could have happened.  In the Dark Forest, yes it was possible and did happen once (as previously mentioned.  However, it would depend on the production staff's readiness with the robotics (for the arms).  There were times where the arms moved long before grabbing somebody.