Best guess is the cap is gone bad.
I can see coolant in the picture and it should be bone dry. Also you might see coolant on the ground just ahead of the rear wheel. I bet you have moisture on the trunk lid above the tank as well. Check the peek window on the tank to check the level.
NEVER use regular coolant. Use the Porsche brand mixed with distilled water 50/50.
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-- 02 Boxster S
-- Black/Black
-- Sideskirts/PSE
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