  As part of the Herschel Guaranteed Time Key Programme CHESS, we present the
discovery of a diffuse gas component in the foreground of the intermediate-mass
protostar OMC-2 FIR 4, located in the Orion A region. Making use of the full
HIFI spectrum of OMC-2 FIR 4 obtained in CHESS, we detected several
ground-state lines from OH+, H2O+, HF, and CH+, all of them seen in absorption
against the dust continuum emission of the protostar's envelope. The lines peak
at a velocity of 9 km/s, which is blue-shifted by 2 km/s with respect to the
systemic velocity of OMC-2 FIR 4 (V = 11.4 km/s). We derived column densities
for each species, as well as an upper limit to the column density of the
undetected H3O+. In order to model and characterise the foreground cloud, we
used the Meudon PDR code to run a homogeneous grid of models that spans a
reasonable range of densities, visual extinctions, cosmic ray ionisation rates
and far-ultraviolet (FUV) radiation fields. The results of our modelling
indicate that the foreground cloud is composed of predominantly neutral diffuse
gas (n(H) = 100 cm-3) and is heavily irradiated by an external source of FUV
that most likely arises from the nearby Trapezium OB association. The cloud is
6 pc thick and bears many similarities with the so-called C+ interface between
Orion-KL and the Trapezium cluster, 2 pc south of OMC-2 FIR 4. We conclude that
the foreground cloud we detected is an extension of the C+ interface seen in
the direction of Orion KL, and interpret it to be the remains of the parental
cloud of OMC-1, which extends from OMC-1 up to OMC-2.
