If CM had presented to a midwife then she would have been flagged up to SS due to the prior removal of her children. Hence, she didn't seem antenatal care.
I don't think anyone would have known she'd given birth if it weren't for the car fire to be honest. I think they were trying to go to Ireland though, and would have tried to settle there, where the perception is the authorities are more likely to keep children with parents.
If the other children have been adopted and she wasn't known to be pregnant then I see no reason for proactive SS monitoring.
Here's a thought - what if they've given the baby to a woman who is desperate for a baby of her own but struggles with infertility or neonatal loss. The sort of woman who buys a reborn doll to cope. She's so desperate for a baby that she agrees they can visit whenever.
She could have presented the baby as her own to friends and family - cryptic pregnancy, miracle baby etc. Undoubtedly she would have bonded with the baby, and phoning the police to admit where the baby is would mean telling her entire social circle that she's spun a web of lies to them.
Wild speculation on my part, and to be honest I think the most likely scenario is the baby has died, but if someone was caring for the baby and didn't come forward it would be a possible scenario.