  Faraday Rotation Measures (RM) should be interpreted with caution because
there could be multiple magneto-ionized medium components that contribute to
the net Faraday rotation along sight-lines. We introduce a simple test using
Galactic diffuse polarised emission that evaluates whether structures evident
in RM observations are associated with distant circumgalactic medium (CGM) or
foreground interstellar medium (ISM). We focus on the Magellanic Leading Arm
region where a clear excess of RM was previously reported. There are two
gaseous objects standing out in this direction: the distant Magellanic Leading
Arm and the nearby Antlia supernova remnant (SNR). We recognized narrow
depolarised filaments in the $2.3\,\rm GHz$ S-band Polarization All Sky Survey
(S-PASS) image that overlaps with the reported RM excess. We suggest that there
is a steep gradient in Faraday rotation in a foreground screen arising from the
Antlia SNR. The estimated strength of the line-of-sight component of the
magnetic field is $B_{\parallel}\sim 5\,\rm\mu G$, assuming that the excess of
RM is entirely an outcome of the magnetized supernova shell. Our analysis
indicates that the overlap between the RM excess and the Magellanic Leading Arm
is only a remarkable coincidence. We suggest for future RM grid studies that
checking Galactic diffuse polarisation maps is a convenient way to identify
local Faraday screens.
