Sorry if this obvious. In episode IV when C-3PO and R2-D2 are in the death star whilst Luke, Han and Ben are "doin the mission", the stormtroopers come into where they are. As they leave one of them stands guard. C-3PO then talks about where Luke and the others might be or whether they are in danger. Why doesn't the stormtrooper ask questions or is he the other side of the slide door. Maybe my eyes decieved me!