Because John doesn't have Sherlock's deduction skills, there are some things he'll simply never know. For instance, he'll never know the truth about what happens the night that he falls asleep on the sofa, after helping Sherlock with a case - even though the evidence will quite literally surround him the next day.
The next morning when he wakes up in the sitting room, John won't put together the clues: a stray thread from Sherlock's coat clinging to the warm fabric, or the fact that the linens' cabinet was left carelessly open in a way that Mrs. Hudson would never leave it. He especially won't remember that his half-sleeping mind had registered, just briefly, a pause in the distant music of the violin, preceding and coinciding with the sudden warmth enveloping him.
John won't find anything particularly remarkable about waking up warm and content, even though the night had been quite cold. Instead, he'll just rub his eyes and shake off the little aches he gets from sleeping on the sofa. He'll curse himself for not getting himself up to bed, but nothing about it will seem all that out of the ordinary.
No, when John wakes up that next morning, he'll assume it was all Mrs. Hudson's doing. After all, who else would bother to cover him with a blanket?
