[Ruth]

Science admitted to Derek that he had taken his cigar and that he knew it was a wrong thing to do (fuelled by alcohol). I doubt very much that he would have taken anybody's cigarette. I think he is just protesting his innocence, because he IS innocent.

(Maybe he reckons that they will think he took the cigs, because they know he took Derek's cigar, so that is why he is protesting so much).