This article investigates who counts as dangerous for immigration control purposes and how states spot and monitor purportedly dangerous people for immigration enforcement measures. By examining Canadian immigration detention law and practice during the early months of the COVID-19 pandemic, the study finds that nearly all danger-based immigration detentions targeted long-term Canadian residents who had typically lost permanent legal status following a criminal conviction. The article argues that a core function of immigration enforcement processes is the removal of supposedly undesirable persons from society and that danger-based detentions are used primarily for post-admission migration control. Furthermore, the study reveals that the surveillance and policing of dangerous individuals largely relies on external police agencies, with immigration officials rarely initiating or managing their own investigations. The findings from this research contribute to the growing body of literature on the overlap between criminal law and immigration law and shows that—in the context of danger-based immigration detention—immigration authorities do not initiate their own investigations, but depend almost exclusively on the work of criminal police forces.