My issue, before it was replaced, was my tank had hidden spider cracks in the back and bottom of the plastic (original 98").
As the cooling system got hot and pressure built up in the system those spider cracks would slightly expand and leak. The leaks were never extreme but slowly, over a few drives the amount of coolant would accumulate enough liquid in my rear trunk (under the removable composite raised floor), that it would eventually find an opening or seam to drip onto the garage floor.
Best advice is to to keep checking just to make sure it's really not leaking anymore.
Good luck!
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98' Boxster
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