Not having had children, I've never been in the habit of monitoring to the degree you would with your own children, like having an ear listening at night even when you're asleep. However I'm not sure if it is a skill at all. I've looked after friends' children including small babies and when doing so I'm very much focussed on them. For me it comes naturally to be focussed on this helpless little creature I'm looking after. Isn't that hard-wired in us somehow - maternal/paternal feelings, which are also activated for any human babies you're interacting with and for furbabies? Well obviously not considering the cases you read about on WS...
Even if distracted for a short time which I suppose might lead you to forget a baby in the car for several minutes or even 10, I cannot imagine going about my day and never remembering the baby again. I do sometimes get distracted in daily life and forget my shopping bags or bike helmet etc and have to go back for them, but I still cannot begin to imagine how this person managed to fall asleep while looking after one baby and then months later forget the other for hours. I would think after the first case, she would have been hypervigilant around the second baby. I just can't imagine how having allowed one to die on her watch, she could have been so utterly unfocused on the other that she forgot about her for hours on end. I was always worried I might go the vet's and forget my furbabies at home or on the bus, but that never happened. Why not? Realising that wasn't the occasion to forget living beings somewhere, all my faculties would be focussed on not doing so! Leaving inanimate objects somewhere is a different kettle of fish because IMO I don't have a emotional connection with them, not the kind you have with children. That's what I can't fathom in this case. How on earth could this woman allow herself to become so distracted TWICE and in the case of her own grandchildren? Sorry for being repetitive but this case leaves me spluttering. MOO JMO
ETA: Sry for maybe not answering all your questions UrsulaWren, but I just got writing and couldn't stop.