  The Leggett-Garg inequality (LGI) distinguishes nonmacrorealistic channels
from macrorealistic ones by constraining the experimental outcomes of the
underlying system. In this work, we propose a class of channels which,
initially, cannot violate the LGI (in the form of the temporal Bell inequality)
but can violate it after the application of stochastic pre- and post-
operations (SPPOs). As a proof-of-principle experiment, we demonstrate the
stochastic pre- and post- operations in an amplitude-damping channel with
photonic qubits. We denote the above phenomenon as hidden nonmacrorealistic
channels. We also discuss the relationship between this hidden
nonmacrorealistic channels (in terms of the temporal Clauser-Horne-Shimony-Holt
(CHSH) inequality) and the strongly nonlocality-breaking channel, which breaks
the hidden spatial CHSH nonlocality for arbitrary states. In general, if the
channel satisfies hidden nonmacrorealism, it is not a strongly CHSH
nonlocality-breaking channel.
