  The production of heavy sterile neutrinos from $\pi^-,K^-$ decay at rest
yields charged leptons with negative helicity (positive for $\pi^+,K^+$). We
obtain the branching ratio for this process and argue that a Stern-Gerlach
filter leads to spatially separated domains of both helicity components with
abundances determined by the branching ratio. Complemented with a search of
monochromatic peaks, this setup can yield both the mass and mixing angles for
sterile neutrinos with masses in the range $3 MeV \lesssim m_s \lesssim 414
MeV$ in next generation high intensity experiments. We also study oscillations
of light Dirac and Majorana sterile neutrinos with $m_s \simeq eV$ produced in
meson decays including decoherence aspects arising from lifetime effects of the
decaying mesons and the stopping distance of the charged lepton in short
baseline experiments. We obtain the transition probability from production to
detection via charged current interactions including these decoherence effects
for 3+1 and 3+2 scenarios, also studying $|\Delta L|=2$ transitions from
$\bar{\nu} \leftrightarrow \nu$ oscillations for Majorana neutrinos and the
impact of these effects on the determination of CP-violating amplitudes. We
argue that decoherence effects are important in current short baseline
accelerator experiments, leading to an underestimate of masses, mixing and
CP-violating angles. At MiniBooNE/SciBooNE we estimate that these effects lead
to an $\sim 15%$ underestimate for sterile neutrino masses $m_s \gtrsim 3
\,\mathrm{eV}$. We argue that reactor and current short baseline accelerator
experiments are fundamentally different and suggest that in future high
intensity experiments with neutrinos produced from $\pi,K$ decay at rest,
stopping the charged leptons on distances much smaller than the decay length of
the parent meson suppresses considerably these decoherence effects.
