How much charge was on the HV battery when you left and how much was on it when you returned? The HV battery normally will not discharge while sitting, at least not very much. And either way even at 0 EV miles the HV battery is far from being fully discharged, the system just won't use it anymore for propulsion until it is charged again.
The HV battery only charges the 12V battery when the car is in READY mode or while the car is being charged. The 12V battery meanwhile is just a standard automotive battery and as obermd said it's not unusual for a 12V battery to be dead after several weeks, especially if the 12V battery is a few years old or if it went through a full discharge sometime in the past. Fortunately nowadays they have those small portable jump start batteries that you can keep in your car, some are so small they fit in the glovebox. They work especially well with hybrids like the Clarity because the portable battery only has to supply enough power to get the 12V system running, then the HV battery will turn the gas engine whenever it needs to be started. And they are safer to use than jumper cables. Even when connected correctly jumper cables can damage the electronics on modern cars.
