  A critical aspect of quantum mechanics is the nonlocal nature of the
wavefunction, a characteristic that may yield unexpected coupling of
nominally-isolated systems. The capacity to detect this coupling can be vital
in many situations, especially those in which its strength is weak. In this
work we address this problem in the context of mesoscopic physics, by
implementing an electron-wave realization of a Fano interferometer using pairs
of coupled quantum point contacts (QPCs). Within this scheme, the discrete
level required for a Fano resonance is provided by pinching off one of the
QPCs, thereby inducing the formation of a quasi-bound state at the center of
its self-consistent potential barrier. Using this system, we demonstrate a form
of \textit{nonequilibrium} Fano resonance (NEFR), in which nonlinear electrical
biasing of the interferometer gives rise to pronounced distortions of its Fano
resonance. Our experimental results are captured well by a quantitative
theoretical model, which considers a system in which a standard two-path Fano
interferometer is coupled to an additional, \textit{intruder}, continuum.
According to this theory, the observed distortions in the Fano resonance arise
\textit{only} in the presence of coupling to the intruder, indicating that the
NEFR provides a sensitive means to infer the presence of weak coupling between
mesoscopic systems.
