Mycroft was co-founder of The Diogenes Club, where the principle rule was Absolute Silence.
The Ancient Greek at the base of the crest, (a direct transcription of Diogenes), is a nod to 'The Greek Interpreter' (published 1893) where The Diogenes Club is first mentioned.
Hidden in the distressed patina of the crest is a reflection of Sherlock in full bloodhound mode.