  Heavy neutral leptons (HNLs) are often among the hypothetical ingredients
behind nonzero neutrino masses. If sufficiently light, they can be produced and
detected in fixed-target-like experiments. We show that if the HNLs belong to a
richer -- but rather generic -- dark sector, their production mechanism can
deviate dramatically from expectations associated to the standard-model weak
interactions. In more detail, we postulate that the dark sector contains an
axion-like particle (ALP) that naturally decays into HNLs. Since ALPs mix with
the pseudoscalar hadrons, the HNL flux might be predominantly associated to the
production of neutral mesons (e.g., $\pi^0$, $\eta$) as opposed to charge
hadrons (e.g., $\pi^\pm$, $K^\pm$). In this case, the physics responsible for
HNL production and decay are not directly related and experiments like DUNE
might be sensitive to HNLs that are too weakly coupled to the standard model to
be produced via weak interactions, as is generically the case of HNLs that play
a direct role in the type-I seesaw mechanism.
