We present the serendipitous discovery of an extended cold gas structure projected close to the brightest cluster galaxy (BCG) of the z=0.045 cluster Abell 3716, from archival integral field spectroscopy. The gas is revealed through narrow Na D line absorption, seen against the stellar light of the BCG, which can be traced for $sim$25 kpc, with a width of 2-4 kpc. The gas is offset to higher velocity than the BCG (by $sim$100 km/s), showing that it is infalling rather than outflowing; the intrinsic linewidth is $sim$80 km/s (FWHM). Very weak H$alpha$ line emission is detected from the structure, and a weak dust absorption feature is suggested from optical imaging, but no stellar counterpart has been identified. We discuss some possible interpretations for the absorber: as a projected low-surface-brightness galaxy, as a stream of gas that was stripped from an infalling cluster galaxy, or as a retired cool-core nebula filament.
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