FWIW, I had the exact same issue. Left the PHEV plugged in overnight, came out and it was completely dead. Per some suggestions on this board, trickle charged the 12v battery for 6 hours, tried again and it booted up normally. Drove it around for 15 minutes in Sport mode to force the ICE to run and charge the traction battery, although it showed full 26 mi range. Everything seemed normal, so put it back in garage. Came out 6 hours later, it was dead again. I DID have the battery saver plus checked, and I had never had it indicated it had needed to charge the 12v battery (which it does if it has been triggered, the next time you boot up). Trickle charged it again so I could get it out of my garage and had Kia have it towed to the dealer. They found the battery failed their load test, but Kia insisted they fully charge the battery and try again. Surprise, after a full charge it passed. No explanation of why it failed with no trouble codes. It is conceivable that the hatch had been slightly ajar overnight, but then why didn't Battery Saver + come on and recharge it, especially since the car was on the charger. Spoke to Kia directly- no joy. Battery has to fail the test twice or they won't replace. And they charge over $200 for their dinky battery if I wanted to pay for it myself (plus labor).
I reluctantly took the car back- nothing else I could do. It's been several months with no reoccurance. I did replace the hatch light with a LED so if it does get left on it will be a much smaller drain. But I also carry a 12000 mah portable car jump starter just in case. The absurdity of driving a PHEV with a GIANT traction battery but being stuck if the dinky 12v battery somehow gets discharged and having to carry a THIRD battery is not lost on me, but it is on Kia. I do love this car, but am very disappointed with Kia's poor engineering around this and their unsatisfactory response to its causing this issue.
2018 White PHEV Premium
2016 White Soul EV