Battery only 3 months old, but dead. (2007)

[lurky]

Hello, I replaced a relatively new battery 3 months ago on my 2007 Boxster.

I went away for two days this weekend, the porsche was in my garage
Yesterday I returned and the car wouldn't start.
When I turn the key I hear it clicking and the lights flash but it wouldn't start. Seems like a dead battery. The lights and radio were not left on.
SO I jumped it from my wife's car and it started. I drove around awhile and I thought it would be fine.
The next morning the same thing happened. Turn the key, hear clicking, lights flashing but wouldn't start.

My battery is only 3 moths old, and I didn't get the cheapie battery either.
Has anyone else experienced a battery problem similar to this one?
What are the common causes?
Is there any way to measure the current drawn from the battery while the car is turned off?
Thanks