Heu, g trouvé une explication sur un site. La voici, en anglais.
Hidden mission " number" in cutscenes
In each intro cutscene for each level, the game director included hidden " numbers" that correspond to the action mission level. The hidden numbers in the cutscenes for each level is listed:Mission 1The number " 1" on the pizza boxMission 2The number " 2" on the billboard at the end of the cutsceneMission 3The burning enemies form the number " 3" at the end of the cutsceneMission 4The hanging banner/flag at the beginning of the cutscene is shaped as the number " 4"Mission 5The blood on the wall at the end of the cutscene forms the number " 5"Mission 6The explosion at the end of the cutscene leaves the number " 6" on the groundMission 7The blood stain on the wall reveals the number " 7" at the end of the cutsceneMission 8The moon in the background has the number " 8" engraved in itMission 9The bullet at the end of the cutscene has " 9mm" marked on itMission 10The blood on the book at the end of the cutscene forms the number " 10"Mission 11The blood on the ground marks the number " 11"Mission 12At the end of the cutscene, the chain attached to the ceiling is holding the number " 12"Mission 13At the end of the cutscene, the wall in the background has the number " 13" engravedMission 14The number " 14" is on the side of the platform, in the pitMission 15The branch at the end of the cutscene shapes the number " 15"Mission 16The number " 16" is lightly scratched on the ground. It\´s easier to see the number at the beginning of the missionMission 17The top of the pillar at the end of the cutscene forms the number " 17"Mission 18A paper containing the number " 18" quickly flies by in the airMission 19As the camera pans to Lady at the beginning of the cutscene, the number " 19" is lightly marked on the groundMission 20The number " 20" is formed by the clouds in the background