What does Acts 23:16 mean?

Paul is incarcerated in the Antonia Fortress next to the temple mount. He's being held because a mob dragged him out of the temple and tried to kill him, and the tribune wants to know why (Acts 21:27–32). So far, the tribune has asked the mob (Acts 21:33–34), let Paul speak to the mob (Acts 22:1–21), almost scourged Paul (Acts 22:23–29), and asked the Sanhedrin (Acts 22:30). Very soon, representatives of the Sanhedrin will approach the tribune and ask him to bring Paul again. The first time, Paul's presence resulted in a melee between the Sadducees and the Pharisees (Acts 23:6–10), but the tribune is getting desperate; he'd probably take the opportunity.

He won't know that the request is a ruse to get Paul out of the barracks and into the open so a group of forty men can assassinate him (Acts 23:12–15). Fortunately, Paul's nephew does know and tells Paul. Paul sends him to the tribune who promptly and wisely sends Paul out of Jerusalem. The tribune is supposed to keep the peace in Jerusalem, making sure the aristocracy doesn't take advantage of the poor. This dispute is getting beyond his authority to control. So, he sends Paul with a military escort to the governor (Acts 23:17–24).

The ease with which Paul's nephew finds him suggests Paul isn't strictly under arrest but is being held for his own safety until the tribune can figure out what's going on. Strangely, this is the only mention of Paul's family other than that his male relations are Pharisees (Acts 23:6).