I've been seeing this frequently. I have a PHEV and I go for days or weeks without running the ICE, often driving in full electric mode for just a few miles a day, but maybe running the headlights in addition to other stuff that requires juice from the 12V battery. Perhaps this is normal for someone who uses the car the way I do. But I recently saw it just two days after I drove 100 miles (and obviously used the ICE on that trip, which should have provided a good opportunity to recharge the 12V battery). On that particular occasion, my electric range seemed to have dropped by several miles overnight while the car was off, although I can't swear to it because I wasn't paying close attention to what the gauge said the day before. I'm wondering how big the drain is on the 12V battery is when the car is off and if there's anything I can do to influence that? I've already disabled the feature that leaves the headlights on for a few minutes after I turn the car off: it wasn't really helpful in my situation.
2018 Niro PHEV Gravity Blue