  The characterization of a quantum system can be complicated by non-ideal
measurement processes. In many systems, the underlying physical measurement is
only sensitive to a single fixed state, complementary outcomes are inferred by
non-detection leaving them vulnerable to out-of-Hilbert space errors such as
particle loss. It is still possible to directly verify the violation of a Bell
inequality, hence witness entanglement of a bipartite state, in the presence of
large vacancy rates using such an incomplete measurement by optimizing the
measurement settings. The scheme is robust against imperfect a priori state
knowledge and also moderate amounts of error in state determination.
