4 ideas
Anne and Marco Conti leave their six-month-old daughter Cora asleep at home with a baby monitor while they attend a dinner party in the adjoining townhouse, taking turns checking on her every half hour. When they come home the baby is gone.
The person best placed to fake an outside crime is the one whose access makes it look impossible for an outsider to have done it. Marco stages the abduction to hide his financial collapse and uses his wife's family money as the target. Once the ransom scheme goes wrong, the plan's secrecy traps him: he cannot seek help without confessing, so a recoverable problem becomes a catastrophe.
When a child disappears, investigators and the public read the parents' private histories as evidence. Anne's postpartum depression and blackout episodes lead the detective, the media, and Anne herself to suspect she harmed her own baby. Once mental illness is disclosed, it becomes the default explanation for gaps in the story and pulls attention away from the real mechanism.
Knowledge of another person's wrongdoing is a tradable asset, and the neighbor who holds it can extract payment instead of reporting it. Cynthia, the hostess, has video of Marco carrying the baby out, and she uses it to blackmail him rather than go to the police. This adds a second hidden conspiracy on top of the first and ties each keeper of a secret to the others.