It was when you discovered that the lock of the despatch-case in Mrs. Inglethorp's bedroom had been forced. 
The lock of the despatch-case did not look like it was tampered with. 
mnli hypothesis: The lock of the despatch-case did not look like it was tampered with.  premise: It was when you discovered that the lock of the despatch-case in Mrs. Inglethorp's bedroom had been forced. 