Why it is a question
THEORY-1151 holds what Guerreiro et al. (2024), LIT-1816, measured: xCOMET flags most injected localized errors and ranks fully detached hallucinations low. But its training data included synthetic detached and oscillatory hallucinations and DEMETR’s diagnostic perturbations, the localized errors were SMAUG perturbations of English targets only, and the hallucinations came from one de-en benchmark. The authors give the synthetic training data as “one plausible hypothesis” for the gains and do not claim that detection generalises. Whether it does is open.
What would answer it
Tests on errors and hallucinations produced by real translation systems in language pairs and kinds the training data did not synthesise (for instance a multilingual hallucination benchmark, or naturally occurring number and entity errors with human labels), showing whether detection stays at the level the synthetic tests report. Results on more synthetic perturbations of the trained kinds do not settle it.