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The Grossman-Nir bound concept is robust, but the common perception of $K_L\to \pi^0\nu\bar\nu < 1.4 \times 10^{-9}$ may be circumvented. Because of theblinding $K\pi2$(3) decay, $K^+$ decay experiments such as E787/E949 and thecurrently running NA62 kinematically exclude the "blinding zone". This offersan opportunity for the currently running KOTO experiment. With no kinematiccontrol, if the KOTO experiment is able to veto all true background, it maydiscover $K_L \to \pi^0 + X^0$, where $X^0$ is an invisible "dark" boson withmass $m_{X^0} \sim m_{\pi^0}$. This could happen with 2015 data at hand, whichsupposedly should allow KOTO to reach nominal GN bound sensitivity. An explicitmodel of gauged $L_\mu - L_\tau$, linked with muon $g-2$ hence with very light$Z'$, is given. Adding vector-like quarks that mix with usual quarks, $W$-bosonloops induce $s\to dZ'$ and $b\to sZ'$ transitions. Besides realizing thescenario, the model provides further illustration of potential impact. Giventhat usual dark bosons linked with the photon are tightly constrained, KOTO hasa unique opportunity to probe the muon-related dark sector.

Loophole in $K \to \pi \nu \bar\nu$ Search \ \& $K_L \to \pi^0 \nu \bar\nu$ Beyond Grossman--Nir Bound