It could be a blocked drain line. The boxsters have concealed tubes that drain water from rain which become blocked overtime, thus resulting in inadequate draining and getting inside the interior.
Of course- you mentioned there was no rain in that period. So-
Perhaps a coolant leak? May be cracks in the coolant reservoir.
Pulled from this thread:
http://986forum.com/forums/general-discussions/30565-coolant-leaking-wet-carpet.html
Or even a blocked AC condensation drain?
Hope that helps.