In terms of an 'outside baby' being stolen from someone, there was a case I followed on here that happened when a woman (unknown to the mother/family) broke in to another woman's house, with a gun, and stole the baby after slashing the mother's throat -- this baby was a newborn too, but with a very noticeable birthmark on her face, commonly called an 'Angel's Kiss'.
Both the mother and baby survived -- the SIL actually identified the baby, the attacker had disguised the birthmark with make-up, which was fairly quickly noticed, within a few days IIRC-- but there was minor suspicion on the mother for various reasons, though generally her story (as she was alive to tell it!) was strongly believed.
Turned out that the baby-snatcher had been pregnant and sadly miscarried (sad, as I said, but most women who go through that don't become criminals and near-killers), although how pregnant she was and when she miscarried was up for debate when I last saw the case. A medical examination did confirm recent pregnancy, however.
This woman was not a friend or even a faux-friend befriended online -- simply, she had seen the 'New Baby!' signs outside the mother's house, apparently from the highway, and had knocked on the door to ask to use the phone and the loo before she produced a gun, mum managed to grab a knife and actually stab her in the leg, then the baby-snatcher got the knife and slashed mum's throat before grabbing the baby.
So, strange things do happen, sadly, and this woman-with-a-gun was clearly not against killing the mother if she had to, and it was only luck that she didn't when she got hold of the knife. It was apparently somewhat impulsive as the baby-snatcher just happened to see the signs, and had gone from there.