We see Sherlock's note sheets that he gave to John and Mary for their wedding right before 221b burst into flames in TFP:
Maybe John gave Sherlock his sheet music back and not a note? The note looked like one piece of paper folded twice, tho. And Molly said Sherlock didn't have to read it right away... So this theory is not very plausible. Still, Sherlock's got his "autographed" sheet music in 221b... *grumble grumble*
----- "The posh boy loves the dominatrix." Context matters.