Prius HV battery SOC 80% flattened my 12v aux. Battery

Today I got home and I left my Enginer kit switched on to charge up the Prius HV battery but I forgot about it. Later on I noticed the Engine was running. I returned to my Prius and the SOC was 82%. I switched off the Enginer switch and let the car run the SOC down to 80% and then I switched off the ignition and locked up the central locking.
About aprox. 6 hours later I could hear the miniBMS buzzer sounding off from my backyard. I went to my Prius and found that it had lost total power. I could not open the doors as they were locked so I used the normal key to open the RHS door. I could not open the rear hatch to get easy access to the 12 volt battery so I had to enter through the back door to climb into the rear hatch area. I found that my 12 volt auxiliary battery was down to 5.51 volts. I quickly disconnected the 12 v aux battery and charged it up with a charger. Has anybody ever heard of a Prius that will use power from the 12 volt aux battery to burn off excess charge of the HV battery. Has this situation ever occurred to anybody else.
Later on I hooked up the charged up 12 volt aux battery and found that the HV battery SOC was down to 62%.