The coolant cap has a part number on it. Latest might be 004 but do a search to confirm. The purge valve has some o-rings and a rubber gasket in it. It can leak and can be replaced.
Water stain and wet insulation under tank. Tank is out.
Purge valve leak.
Any fluid you catch should be the color of you coolant in the tank, so if your finding clear liquid, it's not coolant. The drain hose for the upper area of the tank is in front of the wheel, but if the tank is leaking, then the coolant will be all over that area around the wheel as it runs down. The tank in the trunk is the key to leakage found under the carpet under the tank and towards the tail light.
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