  In many real-world settings, machine learning models need to identify user
inputs that are out-of-domain (OOD) so as to avoid performing wrong actions.
This work focuses on a challenging case of OOD detection, where no labels for
in-domain data are accessible (e.g., no intent labels for the intent
classification task). To this end, we first evaluate different language model
based approaches that predict likelihood for a sequence of tokens. Furthermore,
we propose a novel representation learning based method by combining
unsupervised clustering and contrastive learning so that better data
representations for OOD detection can be learned. Through extensive
experiments, we demonstrate that this method can significantly outperform
likelihood-based methods and can be even competitive to the state-of-the-art
supervised approaches with label information.
