  We report the first evidence of temporal variability in the interstellar Na I
absorption toward HD 47240, which lies behind the Monoceros Loop supernova
remnant (SNR). Analysis of multi-epoch Kitt Peak coud\'{e} feed spectra from
this sightline taken over an eight-year period reveals significant variation in
both the observed column density and the central velocities of the
high-velocity gas components in these spectra. Given the $\sim$1.3 mas
yr$^{-1}$ proper motion of HD 47240 and a SNR distance of 1.6 kpc, this
variation would imply $\sim$10 AU fluctuations within the SNR shell. Similar
variations have been previously reported in the Vela supernova remnant,
suggesting a connection between the expanding supernova remnant gas and the
observed variations. We speculate on the potential nature of the observed
variations toward HD 47240 in the context of the expanding remnant gas
interacting with the ambient ISM.
