This broadcast completes the use of Sir Arthur Conan Doyle's 1924 story "The Adventure of the Illustrious Client".
The method destined to dissolve the body was discovered earlier in the season in The Adventure of the Nutmeg Concoction (2014).
In a flashback scene, showing Sherlock training Kitty in London, Sherlock is teaching Kitty how to remove a security chain from a door, using an elastic band. Given the fact that the only way the security chain could be fastened on the inside of the door is if the occupant was inside, there is no need to subtly remove the chain, as you would be caught breaking into the property by the person inside.