It was when you discovered that the lock of the despatch-case in Mrs. Inglethorp's bedroom had been forced. 
The despatch-case had been forced open and all the important documents were gone. 
mnli hypothesis: The despatch-case had been forced open and all the important documents were gone.  premise: It was when you discovered that the lock of the despatch-case in Mrs. Inglethorp's bedroom had been forced. 