  Motivated by the recent search for a Higgs-Portal Scalar decaying inside the
MicroBooNE detector, we demonstrate that the same search can be used to
constrain Heavy Neutral Leptons. These are gauge-singlet fermions that interact
with the Standard Model by mixing with neutrinos only and could be related to
the origin of neutrino masses. By recasting the results of the MicroBooNE
Collaboration's analysis, we show that, for a Heavy Neutral Lepton that mixes
predominantly with muon-flavored neutrinos, previously unexplored parameter
space can be excluded for masses between 30 and 150 MeV. Additionally, we make
our Monte Carlo tools publicly available.
