  The Laplacian in an unbounded tubular neighbourhood of a hyperplane with
non-Hermitian complex-symmetric Robin-type boundary conditions is investigated
in the limit when the width of the neighbourhood diminishes. We show that the
Laplacian converges in a norm resolvent sense to a self-adjoint Schroedinger
operator in the hyperplane whose potential is expressed solely in terms of the
boundary coupling function. As a consequence, we are able to explain some
peculiar spectral properties of the non-Hermitian Laplacian by known results
for Schroedinger operators.
