Introduction: Retained intracranial foreign bodies in patients alert at a Glasgow Coma Scale (GCS) of 15 are a rare, lethal and medicolegally exposed failure in Indonesian assault casework, yet the share attributable to specific documentable procedural omissions is unquantified. Methods: This retrospective multi-centre cohort examined 45 confirmed retained intracranial foreign bodies identified from 1,240 mild traumatic brain injury presentations (prevalence 3.63%, 95% CI 2.72–4.82) at three trauma centres in North Sumatra, Indonesia, 2019–2023, per STROBE. Diagnosis beyond 24 hours was the pre-specified outcome. With only aggregate data, inference used Fisher exact tests, exact conditional and Firth-penalised odds ratios with profile penalised-likelihood intervals, Holm multiplicity control, and partial identification of the multivariable model across 4,000 admissible joint arrangements. Results: Diagnosis was delayed in 14 of 45 cases (31.1%, 95% CI 19.5–45.7). Failure to palpate the wound bed carried an exact conditional odds ratio of 29.86 (4.79–281.0; p < 0.001) and computed tomography non-adherence 20.88 (3.57–174.7; p < 0.001). As a bedside marker, palpation failure achieved sensitivity 78.6%, specificity 90.3% and area under the curve 0.844; the attributable fraction was 76.3% (50.8–96.1) with an E-value of 11.18. Adjusted odds ratios were bounded, not estimated, because aggregate marginal data cannot identify a multivariable model. Conclusion: Two zero-cost omissions dominate a fatal diagnostic failure; recording wound-bed palpation and the imaging decision is clinically protective and, under KUHAP Articles 133 and 179 and Law No. 17 of 2023, is what a visum et repertum requires.