Yeah, if I find someone promising that seems like they ceased to exist obviously that will ring some bells. But I do think it's possible she could be alive, even if it's very unlikely, so I'm not just focusing on missing persons.
When I think back on the isotope info it suggested that Marlyse and her kids were local but that the middle daughter had only recently arrived. But interestingly, the middle child's isotope map also doesn't point to Mississippi or Louisiana or South Texas. So where was this child? How does she rejoin Rasmussen?
I've always considered the possibility that he killed the mother around the time he supposedly left Texas, probably around mid-1978. But if that's the case, who was the child with while they were in NH?
Maybe it's possible that Rasmussen was able to pawn the child off for a while on a friend or relative. Colorado and Arizona are both in her isotope map zone. Maybe he had a story of either the mom dying tragically or of her up and leaving him. And if he was working on an oil rig, that would be kind of a good cover. You can't take a baby out on an oil rig with you. Plus, that's kind of what he did with Lisa too, so it fits him somewhat.
I estimate for my own purposes that the middle child was born in late 1976. She was estimated to be about a year older than Sarah and we know Sarah was born in late 77. This means if he did pawn her off on someone, she was probably around 18-24 months old at that time.
Maybe he called that person to check in once and a while, promising each time he was almost back on his feet, until they finally said, no, come and get your kid. Or maybe, like with Lisa, they wanted to adopt her and that made him come get her. Presumably, in the case of an adoption, they'd want both parents to sign off, or a death certificate for the mother, and neither would have been an option for him.