Check the condition of the battery to begin with.

Then check the drain on the battery at standstill as if you had left it for a few days.

Disconnect the headunit perm live and then check the drain. It could be this pulling too much out of the battery.

Also unplug the clock as this also has a perm live.

Next, find the main alarm unit and disconnect it. Again check the battery drainage.

The current drawn on the battery should now be 0 (zero) as all battery-needed items are off. (Close the doors when you check this: interior lights )

Next, connect them all back up checking the drain as you add each.
All of these should put very minimal drain on the battery, however, collectively they *MAY be enough to drain it over a few days.

I'm not sure what the limits normally are to be honest (sorry) but I know I used to have the same problem on my S2. Turned out the stereo perm live was wired wrong and was draining too much current from the battery when it was switched off, causing flat battery if left for a week.

If you have access to a plug: buy a trickle-battery charger as people fit to motorhomes and such. Excellent bits of kit: just like a mobile phone charger with croc clips on the end.

Hope this helps...