Sherlock Season 4: 20 Easter Eggs & References In 'The Final Problem'

To break into Sherrinford, both Mycroft and Sherlock disguise themselves as sailors. Mycroft goes for a full make-up job as a swarthy old sea dog while Sherlock just wears a hat and puts on a Scottish accent.

This scene is a reference to the two occasions in the canon when Holmes went incognito as sailors. He briefly donned the guise of a seafaring man in The Sign of the Four and then masqueraded as Captain Basil in Black Peter.