Out-of-home care is intended to shield children from harm, yet some children placed in state guardianship go missing from their placements — a phenomenon poorly understood at population scale. Who these children are, and what becomes of them, has been invisible to researchers and policymakers alike. Using linked administrative data from the New South Wales Human Services Dataset, encompassing over 317,000 care episodes for 47,800 children (2000-2019), this paper provides the first large-scale population-based analysis of children recorded as missing-in-care. We document three key findings. First, missing-in-care episodes are rare (0.5% of all exits) but concentrated among adolescents who enter care late. Second, placement type is the strongest modifiable risk factor. Third, linking care records to nine post-exit administrative datasets, we show that missing-in-care children are profoundly vulnerable. Missing-in-care significantly increases the risk of police contact, victimisation, and assignment to the highest vulnerability service use profile. These findings carry urgent implications for child protection placement policy and integrated service delivery.