  In the non-collinear antiferromagnet Mn$_3$Sn, we compare simultaneous
measurements of the anomalous Hall effect (AHE) and the magneto-optical Kerr
effect (MOKE) through two magnetic phase transitions: the high-temperature
paramagnetic/antiferromagnetic phase transition at the N\'eel temperature ($T_N
\approx$420~K), and a lower-temperature incommensurate magnetic ordering at
$T_1 \approx$270~K. While both the AHE and MOKE are sensitive to the same
underlying symmetries of the antiferromagnetic non-collinear spin order, we
find that the transition temperatures measured by these two techniques
unexpectedly differ by approximately 10~K. Moreover, the applied magnetic field
at which the antiferromagnetic order reverses is significantly larger when
measured by MOKE than when measured by AHE. These results point to a difference
between the bulk and surface magnetic properties of Mn$_3$Sn.
