That's how he got all his information.
3 seconds sound clip from the Clue (1985) movie soundboard.
You can hear this line at 01:12:25.832 in the Blu-ray version of the movie.
Quote context
[...]
- Well, why should we believe you?
- But what I don't understand is why was the cook murdered? She had nothing to do with Mr. Boddy.
- I gathered you all here together because you were all implicated in Mr. Boddy's dastardly blackmail.
- Did none of you deduce that the others were involved, too? What others?
- No.
- That's how he got all his information.
- Before he could blackmail anyone, Mr. Boddy had to discover their guilty secret.
- The cook and Yvette were his accomplices.
- I see. So whoever knew that the cook was involved killed her?
- Yes.
- I know, because I was Mr. Boddy's butler, that the cook had worked for one of you.
[...]