Sherlock Season 4: 20 Easter Eggs & References In 'The Final Problem'
We finally learn here what Mycroft's recurring references to something called Sherrinford have been about. It's actually the top secret facility where Eurus is kept, along with other highly-dangerous criminals labelled "uncontainables".
Sherrinford is taken from the name given to the third Holmes brother in William Baring-Gould's biography of Sherlock Holmes. Baring-Gould chose it as it was Doyle's original name for Sherlock in early drafts of debut novel A Study in Scarlet. For more on Sherrinford, visit our The Six Thatchers easter eggs list.
In the many non-canonical Holmes stories that feature Sherrinford, he is usually depicted as the most intelligent member of the Holmes clan. This distinction is likewise given to Eurus here.