n3elz, has it dead on.
The water pump seals wear and/or the WP bearing is worn causing the seal faces to seperate from time to time. (it's not a lip seal like on your crank or tranny)
If it's truely the WP you will see a residue (greenish-white) streak (leak path)coming from that hole and working it's way downward. Use a mirror or what ever to look up there and see if this is the source.
If so replace the water pump.
(being a OEM water pump engineer for 6 years sorta qualifies me for this question