I cannot believe a child with mobility issues would not be put to bed without a pull-up or diaper. Even when my children were out of diapers/pull-ups at night, I would occasionally find wet pants or pee puddles in between their beds and the bathroom when they couldn't quite make it in time after waking up in the night.
I'm not ruling out Mariah having a bed-wetting accident as the cause of this. Maybe woke up wet and walked out of her room, where EK told her to go back to bed, and when he realized she was wet, he hurriedly tried to get her changed and flipped out either because she was crying or because of the inconvenience. If he'll destroy a car because he was out of smokes, I can't imagine what he'd do if they were out of pull-ups or if he was mad about someone not putting her in a pull-up.
If they got home late that night, I could see skipping out on a pull-up to avoid the risk of waking a sleeping child to put one on. I know we did it lots of times over the years, and then cussed ourselves when we were washing wet sheets the next morning or had a kid come lay in our bed soaking wet and we had to wash two sets of bedding :doh:
ETA: I also think this is why they were so specific about the underwear. As someone mentioned up-thread that it seemed odd that they knew specifically what underwear she was wearing (and that she was only wearing underwear), it was probably because they had just recently seen her in them. I don't think I could tell you what underwear my kids were wearing even when they were little and I got them dressed.