battery dead

Well I went out to start the car this morning and the battery was dead. When I popped the hood to investigate I noticed the negative terminal was wet because it rained last night and stupid me I forgot to cover the car. Could it have drained itself out because it got wet or do I need to start checking out the electrical system. It has started up fine since I got it jumped.

I am guessing that the water could of caused a slow drain. I wouldn't worry about it unless it happens again, just close the hood next time :)