I checked all the fluids, checked the temperature of the AC system (~50 degrees, on a 90 degree+ day) and tried to find a schematic of the AC condensate drain system. I have a disc with the PET catalog, but that did not show the evaporator drain tube from the cabin housing. That system schematic is also not in my Boxster Bentley repair manual.
I followed the plastic tube that is present above the location of the drip. There is a clip that fastens this tube to the underbody panel. This tube goes under the spare tire well, then is seen again near the battery at the firewall. It goes through the firewall, where I assume it connects to the housing holding the evaporator (where condensate would be present for the AC system).
I have not seen any more liquid dripping from the under-car location since my long drive, even though I have used the AC.
So, I conclude that the drip was condensate from a highly worked AC system.