That's the orange safety plug socket that. The white plug plugs into an orange receptacle that tells car orange plug inserted
If you're talking about the white connector, there has to be a mating connector somewhere on the car's harness. I don't remember if it is part of the harness inside the battery case to the ecu, or if it goes to the vehicle harness at the back left corner of the battery case. That is part of the safety interlock circuit - that lets the hybrid control ecu know if the fuse plug is fully installed or not. The car should not go ready if the connector is unplugged - if it does then something is very wrong - fix it before driving! Posted via the PriusChat mobile app.