  Recently Bauer $\textit{et al.}$ arXiv:1909.13022 proposed ANUBIS, an
auxiliary detector to be installed in one of the shafts above the ATLAS or CMS
interaction point, as a tool to search for long-lived particles. Here, we study
the sensitivity of this proposal for long-lived heavy neutral leptons (HNLs) in
both minimal and extended scenarios. We start with the minimal HNL model where
both production and decay of the HNLs are mediated by active-sterile neutrino
mixing, before studying the case of right-handed neutrinos in a left-right
symmetric model. We then consider a $U(1)_{B-L}$ extension of the SM. In this
model HNLs are produced from the decays of the mostly SM-like Higgs boson, via
mixing in the scalar sector of the theory. In all cases, we find that ANUBIS
has sensitivity reach comparable to the proposed MATHUSLA detector. For the
minimal HNL scenario, the contributions from $W$'s decaying to HNLs are more
important at ANUBIS than at MATHUSLA, extending the sensitivity to slightly
larger HNL masses at ANUBIS.
