Seems French police were conducting explosive detection training by placing plastic explosives in luggage passing through the airport and misplaced (lost) a bag and some of the explosives as it moved along the conveyor belt. They say it ended up on one of 90 aircraft that departed de Gaulle, though they do not know which one. They also said it wasn't dangerous because it didn't have a detonator.
http://hosted.ap.org/dynamic/stories/F/FRANCE_LOST_EXPLOSIVES?SITE=DCTMS&SECTION=HOME
http://hosted.ap.org/dynamic/stories/F/FRANCE_LOST_EXPLOSIVES?SITE=DCTMS&SECTION=HOME