When the battery BMS disconnects, it may be possible that the + battery line is not just disconnected, but also grounded. If so, then it can sink a current from another circuit (like a backup battery) to trigger a relay. That way the relay is not drawing a phantom current draw most of the time, while the NC contacts pass the PV current. But as soon as the BMS cuts out, the relay coil gets its signal, opens the contacts, and disconnects the PV array.
You would have to check how the BMS disconnects because "floating open circuit" wouldn't help.