We recently remodeled my in-laws bathroom and, as per code, the entire 20
amp bathroom circuit is protected by a 20A GFCI outlet. Everything was
inspected and functions as it should.

Unfortunately, the GFCI is tripping occasionally when the bathroom vent fan
is turned "off". It never trips when turning on the fan, only when turning
it off. And, it may go a few weeks before it trips, and then suddenly trip
three times in a row. Then it'll work fine again for a few weeks.

I suspected a bad GFCI, so I replaced it with a new one, and also replaced
the switch that controls the fan. It doesn't seem to trip as often now, but
it tripped for me again last night when we were visiting.

The only remaining item seems to be the fan itself, but I'm curious if
there's some other possible cause I might be overlooking? The fan has been
installed for a few months now, so I probably can't return it at this
point.

I thought maybe condensation from moisture draining back through the vent
pipe (it had to be routed up over a beam to get out through the side wall).
But, the tripping doesn't really seem to be related to when the shower was
used recently.

Thoughts?

Anthony