Stars unclassified by SNN clustering algorithm. Stars in four over-dense regions of the sky are highlighted in distinct colors and examined across the same projections as Fig. 1. Spot 1 (red) targets the head of Orion A where Group 5 is recovered. A hole is visible in the proper motion space due to the classified stars in Group 5. Spots 2 and 3 target the OB1b and OB1a associations. Spot 4 targets λ Ori. No over-density in the proper motion space can be identified in these spots. We omit over-densities that correspond to σ Ori and Orion B here because they are already identified in Fig. 1.