Elementary, Potter!
Oh, a Holmesian parody! I loved it how you combined the Snarry and the Holmes/Watson and left us wondering if Snape really IS Holmes, but doesn't want to blow his cover. Or is perhaps fed up with DrWatson'sLupin's literary version of himself - antisocial, dressing up, sniffing dirt, injecting 70% cocaine... I for one wouldn't be too eager to admit I have something in common with that individual ;)
Even in canon there are many similarities between Snape and Holmes, and in this fic you've expounded upon them wonderfully: his lodgings, his brusque manner and superciliousness, his parting ways with DrWatsonLupin, a former confirmed bachelor... Plus, Albus is usually right, and he knows something is afoot!