I see three possibilities:
1.

The gasket between the motor and the gearbox was not good enough

(if generally exists such gasket

- for example by Ford that is a metal gasket against dust and some drops of water make no problems at all) and in the same time there was a leakage on top of the gearbox from the cooling system.

And it has dropped very, very long time.

.

.

2.

We don't have to forget that the condensation also exists and specially in the winter, before the gearbox became worm, the drops are going down, isn't it?
3.

There is also a hole maybe without gasket.

I don't think that you have a problem! For me it's almost normal! It will be worse if the cluch is wet!