Put some talcum powder around the area under the hole in the water pump, if the inner seal goes it leaks water, after passing through all the water pump BEARINGS and washing out the lubricant, if it start rattling seriously stop right away, if you take the belts off you can spin the water pump to see if the bearing grease is getting washed out, a seriously common problem on vauxhalls for some reason, my astra eats a water pump every two years regardless of make, tried GM and all manner of different makes, been through 3 in 5 years!!
I would first check you tank cap, make sure it is getting a good seal otherwise the extra heat when stopped can boil the water.
When the water pump holes start leaking they usually get worse in a hurry.
Kalimex K seal is the best leak stuff I've ever used, and it is permanent!!
tabetha