  Heavy neutral leptons (HNLs) are an attractive minimal extension of the
Standard Model, as is a singlet scalar $s$ mixing with the Higgs boson. If both
are present, it is natural for HNLs to interact with $s$. For a light
singlet,the decay $N\to s\nu$ can dominate over weak HNL decays. We reinterpret
existing constraints on HNL mixing from the DELPHI, CHARM and Belle experiments
for 0.5-100 GeV mass HNLs, taking into account the new decay channel. Although
the constraints are typically weakened, in some cases they can become stronger,
due to observable $s\to\ell^+\ell^-$ decays in the detectors. The method
presented here could be used to recast constraints from other (older)
experiments without resorting to computationally expensive Monte Carlo
simulations. In addition, we update and correct some errors in the analysis of
the original constraints, in the absence of the singlet.
