Possible Coolant Tank Failure

[mnc-i]

Question: If the coolant tank isn't leaking (break or hole in tank) where is the water coming from? My best guess is a bad pressure relief valve on top of the cooolant tank (below the plastic cover with about 10 to 12 screws holding it down)

Here is the background...what do you think?
My local Porsche dealer quoted me $1,000 to replace the tank. A local independent Porsche mechanic quoted me $800 to $850 to replace the tank including new coolant. The independent mechanic told me I should check to see if I had water in the trunk and to check the relief valve. I haven't taken it to be seen by a mechanic yet.

My 1999 Boxster w/193,000 miles gave me the flashing coolant temp light Friday as I left the office for home.
I stopped at Walmart filled the tank with distilled water (approximately 1.5 quarts).
Over the weekend and Monday at lunch I poured the rest of the 1 gallon jug in the coolant tank.
I never had any water on the ground under the rear of the car. I did have condensation in the trunk right over the dip stick, oil filler cap & coolant cap.
This evening after I drove home and parked the car I had coolant running from under the car from below the coolant tank.
I took the carpet & padding out. There was no water on the trunk floor and no signs that there ever was any water in the trunk below the carpet (totally dry).
I also saw no signs of a leak from my coolant tank.

What are your thoughts/recommendations?

MNC-I