"Where did that come from?" Holmes inquires, quirking one eyebrow. It was only a cheap wall piano, but naturally, he noticed it.
"I took up playing while you were. . .away." Watson's real musical inspiration is clear, though, from the glance he gives to Mary's photograph on the mantlepiece.
Holmes's face clouds at this, closing his friend out for a moment. Then one of his sudden grins splits the wall between them. He reaches out and pulls Watson's shoulder to his, Holmes's closest approximation of an embrace.
When Holmes left the room, Watson knew it was to get his violin.