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 1: The number "1" on the pizza box
Mission 2: The number "2" on the billboard at the end of the cutscene
Mission 3: The burning enemies form the number "3" at the end of the cutscene
Mission 4: The hanging banner/flag at the beginning of the cutscene is shaped as the number "4"
Mission 5: The blood on the wall at the end of the cutscene forms the number "5"
Mission 6: The explosion at the end of the cutscene leaves the number "6" on the ground
Mission 7: The blood stain on the wall reveals the number "7" at the end of the cutscene
Mission 8: The moon in the background has the number "8" engraved in it
Mission 9: The bullet at the end of the cutscene has "9mm" marked on it
Mission 10: The blood on the book at the end of the cutscene forms the number "10"
Mission 11: The blood on the ground marks the number "11"
Mission 12: At the end of the cutscene, the chain attached to the ceiling is holding the number "12"
Mission 13: At the end of the cutscene, the wall in the background has the number "13" engraved
Mission 14: The number "14" is on the side of the platform, in the pit
Mission 15: The branch at the end of the cutscene shapes the number "15"
Mission 16: The number "16" is lightly scratched on the ground. It's easier to see the number at the beginning of the mission
Mission 17: The top of the pillar at the end of the cutscene forms the number "17"
Mission 18: A paper containing the number "18" quickly flies by in the air
Mission 19: As the camera pans to Lady at the beginning of the cutscene, the number "19" is lightly marked on the ground
Mission 20: The number "20" is formed by the clouds in the background.