He lives in Hell's Kitchen so I'm not sure about neighbors.. maybe she knows someone in his building (or him hers) and they bump into the other that way?
Yeah, he doesn't do relationships either. Had he not already had that one occasional hook up (who is also a detective) that would have worked, but he'd feel like a complete man whore if he slept around with more than one person on the force. We can do a past fling, though.. Someone he managed to remain friends with?