I think the forum limits what you can post until you've been a member for a while. Too bad, because I would have like to see/hear your video.
You said it was plugged in, but how long had it been since you last turned the car on before this happened?
Are you certain that it said PB? My 2018 PHEV owner's manual lists several PD fuses, but nothing for PB.
Keep in mind that the sound you heard might have been a symptom, rather than a cause. For example, if the rear door isn't all the way closed or the vanity light over one of the visors is left on, then this could deplete the 12 V battery. If BS+ is enabled (and you indicated that it is), then the manual indicates that the computer will make a limited effort to maintain the 12V battery with energy stored in the traction battery, but eventually (after so many minutes and attempts) it will give up, potentially allowing the 12 V battery to go flat. If you happen to come along with the key fob in your pocket at just the right time in that process, it's easy to imagine that maybe there was still enough power in the 12V battery for the key fob sensor to detect the key fob and try to trigger one of the relays to wake up the car, but maybe the voltage in the 12V battery was so low at that point that the relay couldn't "pull in" and wound up making the noise you heard instead.
A variant on this idea is that the battery was depleted not because you left a light on, but because of some fault in the car's logic that prevented it from going in to sleep mode, or because your 12V battery is a dud that won't hold a charge. My office is located over the garage and I've often wondered if walking in to my office with the key fob in my pocket wakes up the car and induces an additional drain on the 12V battery.
One other thing to evaluate: it sounds like you drive your PHEV the way I do (I drive for weeks at a time in EV mode exclusively, which means that the ICE doesn't run and the only way to recharge the 12V battery is with the Battery Saver Plus system). If you really have the Battery Saver Plus system enabled, then you should occasionally see a popup message on the dash when you turn the car on notifying you that BS+ ran while the car was off. It typically displays at about the time that the startup tune is ending, and it also makes a chime sound of its own, but it only displays for about 2 seconds. Have you been seeing that notification? If not, maybe it's not really enabled, or not really working, or else maybe Kia changed the behavior for the 2019 model (but I doubt that the behavior changed for 2019).
2018 Niro PHEV Gravity Blue