I have a Hunter in-ground irrigation system.
In one of the zones, I have noticed a slow trickle drip down my
driveway from one of the sprinklers that continues even after the
system has been "off" for several days.

My guess is that perhaps the valve controlling that zone has a very
slow leak which would be consistent with the fact that this sprinkler
is at the lowest grade of the 4 sprinkler heads in the zone.

Does this diagnosis make sense?
If so what typically causes such a leak and can it be repaired? Or do
I need to go buy a new valve?

Thanks,
Jeff