  The $A$V$_3$Sb$_5$ ($A=$ K, Rb, Cs) family of Kagome metals hosts
unconventional charge density wave order whose nature is still an open puzzle.
Accumulated evidences point to a time-reversal symmetry breaking orbital flux
phase that carries loop currents. Such an order may support anomalous Hall
effect. However, the polar Kerr effect measurements that probe the a.c.
anomalous Hall conductivity seems to have yielded contradictory results. We
first argue on symmetry grounds that some previously proposed orbital flux
order, most notably the one with Star-of-David distortion, shall not give rise
to anomalous Hall or polar Kerr effects. We further take the tri-hexagonal
orbital flux phase as an exemplary Kagome flux order that does exhibit
anomalous Hall response, and show that the Kerr rotation angle at two relevant
experimental optical frequencies generally reaches microradians to
sub-milliradians levels. A particularly sharp resonance enhancement is observed
at around $\hbar \omega =1$ eV, suggesting exceedingly large Kerr rotation at
the corresponding probing frequencies not yet accessed by previous experiments.
Our study can help to interpret the Kerr measurements on $A$V$_3$Sb$_5$ and to
eventually resolve the nature of their CDW order.
