Visible crew/equipment: When Mrs. Peacock turns on the cellar lights, a piece of lighting equipment is visible to the left of the screen. (00:47:45)
Visible crew/equipment: Towards the end of the film, when the butler is explaining 'whodunit', as he stands at the front door, quite clearly at the top of the screen a microphone can be seen following him back and forth.
Visible crew/equipment: After opening the doors to the lounge to let Col. Mustard and Miss Scarlet out Mrs. White says "How did you get in? The doors were locked." After this line, for the remainder of the scene, on the left side of the wall beside the chair the reflection of a crew member and the boom mic pole can be seen moving around.
Visible crew/equipment: When the cop is driving past the broken down vehicle, a crew member's arm can be seen reflected on the car. (00:49:35)
Answer: As Wadsworth explained to everybody, when he saw the motorist at the door, he didn't throw the key away but put it back in his coat pocket. While everyone's attention was on the motorist, Col. Mustard took the key from Wadsworth pocket and replaced it with a different key.