Battery completely dead

[Joe B]

Buy an AGM battery and disconnect the negative terminal (or put it on a trickle charger) if the car won't be driven for more than a week.
End of problem !

[Fatbloke]

Looks like the big charger is charging the battery. I will leave it on overnight on normal charge rate and see if it is fully charged tomorrow.

Is there any way I can check what's draining it?

[Joe B]

All Boxsters drain the battery more than most cars. Alarm system? I don't know, but it's normal. And just because your battery comes up to a full charge doesn't mean it will hold it, especially if it's been drawn all the way down before. Automotive starting batteries don't like being fully discharged. You need to take the battery into a shop or auto parts store and have them load test it.