I bought a brand new battery for my C70 D5 and after a two week holiday, I came back to find it dead even though it was fully charged before I left.
I worked it out that having the alarm engaged drains power from the battery and over two weeks it had drained it completely.
If your battery is a few years old maybe it can't hold as much charge and it drains it quicker.
I just leave the alarm off if I'm not using the car and rely on the keyless entry for security.
HTH.
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