Circuit Description - DTC P3100 - Information Code 142: 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.
The HV ECU checks the ST signal and detects malfunction. If the ST signal has +B short, the ST will always be ON. Therefore, even with the IG ON when starting the THS, the system will start. To prevent this from happening, the HV ECU detects the malfunction of the ST signal.