  If leptonic unitarity is violated by new physics at an energy scale much
lower than the electroweak scale, which we call low-scale unitarity violation,
it has different characteristic features from those expected in unitarity
violation at high-energy scales. They include maintaining flavor universality
and absence of zero-distance flavor transition. We present a framework for
testing such unitarity violation at low energies by neutrino oscillation
experiments. Starting from the unitary 3 active plus $N$ (arbitrary integer)
sterile neutrino model we show that by restricting the active-sterile and
sterile-sterile neutrino mass squared differences to $\gtrsim$ 0.1 eV$^2$ the
oscillation probability in the $(3+N)$ model becomes insensitive to details of
the sterile sector, providing a nearly model-independent framework for testing
low-scale unitarity violation. Yet, the presence of the sterile sector leaves
trace as a constant probability leaking term, which distinguishes low-scale
unitarity violation from the high-scale one. The non-unitary mixing matrix in
the active neutrino subspace is common for the both cases. We analyze how
severely the unitarity violation can be constrained in $\nu_{e}$-row by taking
a JUNO-like setting to simulate medium baseline reactor experiments. Possible
modification of the features of the $(3+N)$ model due to matter effect is
discussed to first order in the matter potential.
