You have something draining the battery. Check that the radio is not drawing more current than it should be. Also go through the alarm and imobiliser wiring to check that circuit.

I used to have this problem on my S2 and auto electricians said they found nothing wrong. In the end I took it to my friends garage and we took an initial current reading (think it was about 0.8) then disconnected the power to the alarm, then the seperate immobiliser taking readings evertime we disconnected something. Then the clock. It still read about 0.3/0.4 at this point which was about right on the drop we had for what we had disconnected (0.2/0.3) but still had something draining power. I then fully disconnected the stereo and the power drain was 0.
My stereo permanent and accessory lives were both connected to the same wire so it was correctly drawing power to remember the stations, time, etc but drawing it from both wires so doubling the draw.

After correctly fitting the stereo wiring and reconnecting digi clock and alarm, immob the drain was around the 0.5/0.6 mark which was acceptable for what was perm live.