  A variety of new physics scenarios allow for neutrinos to up-scatter into a
heavy neutral lepton state. For a range of couplings and neutrino energies, the
heavy neutrino may travel some distance before decaying to visible final
states. When both the up-scattering and decay occur within the detector volume,
these "double bang" events produce distinctive phenomenology with very low
background. In this work, we first consider the current sensitivity at
Super-Kamiokande via the atmospheric neutrino flux, and find current data may
already provide new constraints. We then examine projected future sensitivity
at DUNE and Hyper-Kamiokande, including both atmospheric and beam flux
contributions to double-bang signals.
