Circuit Description - DTC P1600 - Information Code 117: Notes
WARNING: This page is about a different car, the 2002 Toyota Prius. However, it is still accessible from the selected car via links, so may be relevant.
Since the ECU back-up power source is used for DTC and freeze frame data memory, the back-up power source (BATT) continues to be supplied to the HV ECU even though the ignition switch is turned OFF.