i would start by getting the battery checked as it sounds like its flat.
make sure there are no lights left on in the car,on mine i was having trouble starting it,horrible squeal from the starter,when the missus gets into the car she pulled down the visor and the light was on,it was bloody hot.so aint got a scooby on how long it had been left on for,but starts a lot better now,but i do suspect my battery is on its last legs.
Hi I wouldn't think its the battery in itself, you probably have something(electrical fault) that could be draining the battery.Battery is just over 3 years and should still have a couple more years of life left in it.
Water in the alarm module (which is stupidly mounted in the N/S/F wheel arch) would be my bet.
take a looksie within the door gators for any frayed/broken wires.
Have you checked the alarm module yet?