You can't lose a quest items, it will be bound to your character's inventory, and you won't be able to sell or destroy it unless you pass a prompt check. The way it could have disappeared from your inventory is that you simply gave it to the robot. I think if you sell or destroy quest items without turning them in they will reappear at the same location from which they were picked up. I know I did that with an Orsinium treasure for the museum.