Martin is throwin gang signs but it looks like Ben is just telling us all to live long and prosper! <-- Actually, long story short, but Ben's sign means "don't forget to be awesome." People need to read up on nerdfighters, and our signs, yo!
Benedict has known Una Stubbs (Mrs. Hudson) his entire life because she is a close friend of his mother. So the affection Sherlock shows Mrs. H is completely real and completely Benedict's doing (Moffat didn't write it in the script initially).