  Lepton-number violation can be induced by the exchange of an on-shell
Majorana neutrino $N$ in semileptonic $|\Delta L|=2$ decays of the $B_s$ meson,
$B_s^0 \to P^-\pi^-\mu^+ \mu^+$ with $P=K, D_s$. We investigate the production
of such a heavy sterile neutrino through these four-body $\mu^+\mu^+$ channels
and explore the sensitivity that can be reached at the LHCb and CMS
experiments. For heavy neutrino lifetimes of $\tau_N$ = [1, 100, 1000] ps and
integrated luminosities collected of 10 and 50 fb${}^{-1}$ at the LHCb and 30,
300, and 3000 fb${}^{-1}$ at the CMS, we find a significant sensitivity on
branching fractions of the orders ${\rm BR}(B_s^0 \to K^- \pi^-\mu^+ \mu^+)
\lesssim \mathcal{O}(10^{-9} - 10^{-8})$ and ${\rm BR}(B_s^0 \to D_s^-
\pi^-\mu^+ \mu^+) \lesssim \mathcal{O}(10^{-8} - 10^{-7})$. In the
kinematically allowed mass ranges of $m_N \in [0.25,4.77]$ GeV and $m_N \in
[0.25,3.29]$ GeV, respectively, we exclude regions on the parameter space
($m_N,|V_{\mu N}|^{2}$) associated with the heavy neutrino, which could
slightly improve the limits from $B^- \to \pi^+\mu^-\mu^-$ (LHCb).
