Sorry to be the bearer of bad news but it sounds like you may have a water pump that is on the fritz. Your water pump has a small "weep" hole underneath and it is entirely possible that it's leaking only when your car is not running. Mine did the same thing... The dealer couldn't find my leak after pressure testing. It was only after they let it sit overnight that they were able to locate the leak at the water pump. If this is your problem and you replace the pump, make sure you get the hose that goes from the pump to the block (Mine was part number 103-203-00-82) as the two ends of the hose are totally different sizes. At the dealer expect to pay for 6 hours of labor to replace it as there is a number of things that have to to be moved and removed. At the dealer the water pump costs about $119.00 US.
Hope that helps/David