  Heavy sterile-like neutrinos may be produced resonantly from the decay of
pseudoscalar mesons and may decay into several different channels in a cascade
$\Phi \rightarrow L^\alpha \nu_h;\nu_h\rightarrow \{X\}$. In general these are
rare events with displaced vertices. We provide a non-perturbative and
manifestly unitary framework that describes the cascade decay and yields the
space-time evolution of the probabilities for sterile neutrinos, final states
and the total number of events at a far detector. The results are general,
valid for Dirac or Majorana neutrinos and only input the total decay rates and
branching ratios for the production and decay channels. We apply the general
results to two examples of "visible" decay: i) $K^+\rightarrow e^+
\nu_h\rightarrow (e^+) e^+ e^- \nu_e$ via a standard model charged current
vertex and ii) the radiative decay $K^+\rightarrow \mu^+ \nu_h \rightarrow
(\mu^+) \nu_a \gamma$. For this latter cascade process we find substantial
corrections to previous assessments within the parameter space argued to solve
the anomalous excess of electron-like events at MiniBooNE. These large
corrections may help relieve the tension with recent experimental bounds on
radiative decays of heavy sterile neutrinos.
