  An observation of any lepton number violating process will undoubtedly point
towards the existence of new physics and indirectly to the clear Majorana
nature of the exchanged fermion. In this work, we explore the potential of a
minimal extension of the Standard Model via heavy sterile fermions with masses
in the $[ 0.1 - 10]$ GeV range concerning an extensive array of "neutrinoless"
meson and tau decay processes. We assume that the Majorana neutrinos are
produced on-shell, and focus on three-body decays. We conduct an update on the
bounds on the active-sterile mixing elements, $|U_{\ell_\alpha 4} U_{\ell_\beta
4}|$, taking into account the most recent experimental bounds (and constraints)
and new theoretical inputs, as well as the effects of a finite detector,
imposing that the heavy neutrino decay within the detector. This allows to
establish up-to-date comprehensive constraints on the sterile fermion parameter
space. Our results suggest that the branching fractions of several decays are
close to current sensitivities (likely within reach of future facilities), some
being already in conflict with current data (as is the case of $K^+ \to
\ell_\alpha^+ \ell_\beta^+ \pi^-$, and $\tau^- \to \mu^+ \pi^- \pi^-$). We use
these processes to extract constraints on all entries of an enlarged definition
of a $3\times 3$ "effective" Majorana neutrino mass matrix $m_{\nu}^{\alpha
\beta}$.
