Sherlock Season 4: 20 Easter Eggs & References In 'The Final Problem'
10. Vatican Cameos
When John realises something is wrong at Sherrinford, he tries to let Sherlock know by sending him the message "Vatican cameos."
In The Hound of the Baskervilles, Holmes says at one point: “I was exceedingly preoccupied by that little affair of the Vatican cameos, and in my anxiety to oblige the Pope I lost touch with several interesting English cases."
In Sherlock, the phrase has been reattributed as John and Sherlock's codeword for when things are getting serious. Sherlock previously said the warning to John in A Scandal in Belgravia and The Sign of Three.