Tracked down open circuit on 2nd floor Nac 1. Removed two devices in the hall just outside the elevators. On side had power but no EOL, the other side had EOL but no power, so the issue was coming from in between the two devices. Accessed two units, and found disconnected speaker in Unit 256. Reconnected device and open circuit trouble cleared from the panel. System is online with one trouble remaining for a smoke detector.