Mine does the same and I always assumed that it was the alarm.
Next time you replace the battery, get an Ultima Yellow, Red or Blue Top deep cycle if it's going to sit unused for weeks at a time.
Also cold, damp weather drains them by moisture tracking and shorting between the battery terminals which is why people recommend insulating them with a finger full of petroleum jelly like Vaseline.
Another tip is that if the battery is dead and when you plug it in, your charger won't charge the battery without tripping into trickle mode as if it's damaged, just keep re-connecting it and letting it trip and after a few attempts the battery voltage will increase to the point of charging fully in one go.
My Halford's automatic charger does this as it needs the battery to be above x volts ( typically 11.3) or it thinks it's beyond repair and won't bother.
Lead acid batteries can be 'damaged' by allowing them to drain completely so if possible, never let it go flat before re-charging or it will develop a coating on the lead inside reducing it's performance.
Optima batteries are designed to be more resistant to this effect.
The Red Top is best for keeping it's voltage up when not used for long periods but most car enthusiasts buy Yellow Tops as they have a bit more deep cycling capacity. Blue Tops are typically used in boats and RV's that have more accessories running off them and they can be drained further without damage.
Halford's Calcium are a cheap but good battery with a 4 year guarantee the last time I bought one. They still go flat in a week without being used though!