Allen would love a connection with the police, someone who could tell him when a supernatural body is found or when someone goes missing, particularly changelings. A little give-and-take~ He sometimes drops by the Sanctuary for food and supplies, so maybe that's how Sherlock first notices him? Or maybe he hears of Allen's exploits and goes looking to gather intel. We can assume all this as CR if you like, and go from there! Maybe a case will come up that will send Sherlock back to Allen and his homeless network