  We consider a scenario with unitary mixing of the three light standard
neutrinos and a non-unitary mixing contribution of a heavier massive neutrino
that can generate short-baseline neutrino oscillations. We show that this
scenario predicts constant flavor-changing probabilities at short-baseline
distances. Therefore, it cannot explain the spectral distortions observed in
the LSND and MiniBooNE appearance experiments. On the other hand, the survival
probabilities oscillate as functions of $L/E$ and could explain oscillations in
short-baseline disappearance experiments. We also derive the bounds on the
mixing parameters from the existing short-baseline neutrino oscillation data.
