  Belle II recently reported the first measurement of $B^+\to
K^++\mathrm{inv}$, which is $2.8\sigma$ above the Standard Model prediction. We
explore the available parameter space of new physics within Standard Model
effective field theory extended by sterile neutrinos ($\nu$SMEFT) and provide
predictions for the other $B\to K^{(\star)}+\mathrm{inv}$ decay modes and
invisible $B_s$ decays. We also briefly comment on charged current decays $B\to
D^{(\star)}\ell\bar\nu$ and possible ultraviolet completions of the relevant
$\nu$SMEFT operators.
