  A sandwich detector composed of scintillator and steel-covered lead layers
was introduced in the fixed-target COMPASS experiment at CERN for vetoing
events not completely covered by the two-stage magnetic spectrometer.
Wavelength shifting fibres glued into grooves in the scintillator tiles serve
for fast read-out. Minimum ionizing particles impinging on the $2 \textrm{m}
\times 2 \textrm{m}$ detector outside of a central hole, sparing the
spectrometer's entry, are detected with a probability of 98%. The response to
charged particles and photons is modeled in detail in Monte Carlo calculations.
Figures of merit of the veto trigger in $190 \textrm{GeV}/c$ $\pi^- + p$ (or
nucleus) experiments are an enrichment of exclusive events in the recorded data
by a factor of 3.5 and a false-veto probability of 1%.
