If the battery was fully charged when you locked it up and left it and if it was on a trickle charger during that time and still didn't hold a charge than I'd say faulty battery or charger. As reported here before the car uses a lot juice when locked and I'm not terribly convinced that Porsche delivers them with a fully charged battery to start with. Routine driving alone may not get the battery fully charged if sufficent speed and distance is not achieved as the Owner's Manual warns.

You need to promise the Cayman that trip to Spa for the F1 race...