Could you imagine seeing someone just tapping out a conversation? With no one? Who’s that guy talking to, is this just a nervous habit? Then realizing that someone else might actually be tapping back… There’s more to this guy than meets the eye.
There actually is one other character who knows Morse (without learning it within the story context, which a few likely will!). John Watson knows it from his time serving, and used it a few times on cases during his time with Sherlock. How he puts this skill to use with the tiny bros remains to be seen!