Non-verbal, special needs sister at the center of a nasty mystery mess.

My sister is 23, has Down Syndrome and Autism, is non-verbal, wears a helmet that protects her head and face and still wears a diaper. This past July 11th was my late brother's birthday. On that morning she woke my parents up, my dad went to her room and there was poop everywhere. On the headboard of the bed, on the mattress, the floor, some on the walls. Weirdly enough, the poop didn't really smell, and it didn't smear. Like, they would wipe it off the walls and it would just come off without any noticeable residue. They went to clean my sister up, expecting her to have poop all over her, but they checked her hands and her feet and they were all clean. Her hands didn't even smell (my sister doesn't really "play" with poop like that anyway). They checked her diaper and besides some pee it was completely clean. No poop at all. Even her butt was poop-free. She didn't need to be wiped. My dad said that it was like a really immature, disgusting ghost visited my sister's room and left a mess. Or that a really nice ghost came and cleaned her up after the mess was made. It sounds completely ridiculous but they have NO idea how to explain it. Neither of my parents has ever sleep-walked. Can anyone offer any suggestions as to what might have happened?

Edit: We don't own any pets nor do we have any issues with stray animals finding their way into our home. My sister didn't get into any old diapers. She doesn't possess the motor skills or coordination to get rid of all the evidence that would've explained the event. Theoretically she could change her pants but it would be a big effort for her. Changing her diaper, wiping her butt, washing her hands clean and putting on clean clothes all by herself is genuinely inconceivable.

Edit #2 (no pun intended lol): it was definitely poop. It wasn't vomit (either from brown foods or from a constipation buildup), paint, watercolors, chocolate, or anything else that might be poop-colored. My parents know what they cleaned up and that's why the whole event is so confusing. It's been a week and we've talked about it every day trying to figure out what could've happened.