  The existence of heavy neutrinos is the most prominent feature of type-I
seesaw-like mechanisms, which naturally explain tiny masses of observed
neutrinos. Various experiments have been conducted to search for heavy
neutrinos, with meson decays sensitive to heavy neutrinos lighter than $B$
mesons and high energy collider experiments sensitive to the ones with masses
larger than $\mathcal{O}(0.1)$ TeV. In between, there is a mass gap uncovered
by these two kinds of experiments, and by this work, we investigate the 3-15
GeV neutrino searches at future $Z$-factories via the process
$e^{-}e^{+}\rightarrow Z \rightarrow \nu N \rightarrow \nu \ell\bar{q}q'$ with
$\ell=e,\mu$. Based on the simulation of signal and background events, we find
that the signals appear as monojet events owning to the large Lorentz boost of
the heavy neutrinos. Furthermore, the substructures of such monojet are
essential to reconstruct the signal and suppress the background. The upper
bounds on the cross section of the signal process and the mixing parameter ${|
V_{\ell N}|}^2$ are obtained. Compared with other experiments, the monojet
method at future $Z$-factories will be able to fill the gap around this mass
range.
