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We point out kaon decays to multiple charged leptons as a novel probe oflight dark particles $X$. Previously neglected channels, such as $K^+\to \pi^+\left(XX \to 2(e^+e^-)\right)$, $K_L\to \pi^0 \left( XX \to 2(e^+e^-)\right)$,and $K_S\to \left(XX \to 2(e^+e^-)\right)$ may have very large rates, exceedingnot only the Standard Model expectations but also possible backgrounds, such asDalitz decays of neutral pions. We apply this idea to dark sector models wherethe production of dark Higgses or heavy neutral leptons leads to final stateswith several visible dark photons. We also investigate a recently proposedmodel of an MeV-scale QCD axion, where the rates for kaon decays to multipleaxion states are large due to the non-linear interactions of the axion with thelight mesons. In addition, we point out new probes of this axion in piondecays, such as the single production of $a$ in $\pi^+ \to \nu \left((e^+)^*\to e^+ a\to e^+e^+e^-\right)$, double production in pion capture $\pi^-+(p\text{ or D})\to aa + (n\text{ or }nn) \to 2(e^+e^-) + (n\text{ or }nn)$, aswell as $\pi^0 \to aaa\to 3(e^+e^-)$. The latter decay is fixed at$\mathcal{B}(\pi^0\to a a a) = 1.0 \times 10^{-3}$ for a $17$ MeV axion.

Novel multi-lepton signatures of dark sectors in light meson decays