  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 the
blinding $K\pi2$(3) decay, $K^+$ decay experiments such as E787/E949 and the
currently running NA62 kinematically exclude the "blinding zone". This offers
an opportunity for the currently running KOTO experiment. With no kinematic
control, if the KOTO experiment is able to veto all true background, it may
discover $K_L \to \pi^0 + X^0$, where $X^0$ is an invisible "dark" boson with
mass $m_{X^0} \sim m_{\pi^0}$. This could happen with 2015 data at hand, which
supposedly should allow KOTO to reach nominal GN bound sensitivity. An explicit
model 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, $W$-boson
loops induce $s\to dZ'$ and $b\to sZ'$ transitions. Besides realizing the
scenario, the model provides further illustration of potential impact. Given
that usual dark bosons linked with the photon are tightly constrained, KOTO has
a unique opportunity to probe the muon-related dark sector.
