if your tri-metric shows NO discharge at night
why do you think, there is a power back feed, when there is None
i'm guessing the ambient light is producing enough Milli-amps of power from the panel, to run the fan at night, but not enough to turn on the controller for charging the batteries, a tiny DC Fan will might run on ten volts and 50ma of current
connect your fan, cover the panels with a blanket or cardboard
and see what happens
if you can't hear it, and its running from the panels, not the battery
it would make no difference to me if it ran full time and never stopped
as long as the power did not come from the batteries
