Modern object detectors are largely confined to a "closed-world" assumption, limiting them to a predefined set of classes and posing risks when encountering novel objects in real-world scenarios. While open-set detection methods aim to address this by identifying such instances as 'Unknown', this is often insufficient. Rather than treating all unknowns as a single class, assigning them more descriptive subcategories can enhance decision-making in safety-critical contexts. For example, identifying an object as an 'Unknown Animal' (requiring an urgent stop) versus 'Unknown Debris' (requiring a safe lane change) is far more useful than just 'Unknown' in autonomous driving. To bridge this gap, we introduce TARO, a novel detection framework that not only identifies unknown objects but also classifies them into coarse parent categories within a semantic hierarchy. TARO employs a unique architecture with a sparsemax-based head for modeling objectness, a hierarchy-guided relabeling component that provides auxiliary supervision, and a classification module that learns hierarchical relationships. Experiments show TARO can categorize up to 29.9% of unknowns into meaningful coarse classes, significantly reduce confusion between unknown and known classes, and achieve competitive performance in both unknown recall and known mAP. Code will be made available.
