  The experimental verification of quantum features, such as entanglement, at
large scales is extremely challenging because of environment-induced
decoherence. Indeed, measurement techniques for demonstrating the quantumness
of multiparticle systems in the presence of losses are difficult to define and,
if not sufficiently accurate, they provide wrong conclusions. We present a Bell
test where one photon of an entangled pair is amplified and then detected by
threshold detectors, whose signals undergo postselection. The amplification is
performed by a classical machine, which produces a fully separable micro-macro
state. However, by adopting such a technique, one can surprisingly observe a
violation of the CHSH inequality. This is due to the fact that ignoring the
detection loophole, opened by the postselection and the system losses, can lead
to misinterpretations, such as claiming the micro-macro entanglement in a setup
where evidently there is not. By using threshold detectors and postselection,
one can only infer the entanglement of the initial pair of photons, so
micro-micro entanglement, as it is further confirmed by the violation of a
non-separability criterion for bipartite systems. How to detect photonic
micro-macro entanglement in the presence of losses with currently available
technology remains an open question.
