In a gentrifying neighborhood of San Francisco, a couple exit their cab and head toward an apartment, rolling suitcases behind them. Unbeknownst to them, a private investigator by the name of Michael Joffe sits in his parked car just across the street, discreetly snapping pictures. This is not a divorce case waiting to happen or an international spy caper. Nothing that salacious or mysterious. It is instead an episode that provides a window into how bitter the feud between struggling tenants and home-sharing websites like Airbnb Inc. has become. Joffe works for a tenant lawyer who in turns represents a family that was evicted from their apartment — the one that the couple was entering that day.