Why did my battery go flat ?

[Willy]

Unplugging the siren module (and sealing the connector) is indeed the best thing to do.
If not, the canbus can still try to cpmmunicate with it leading to (some) unnecessary power loss.
It may also lead to canbus "stress" which is not what you want.
When we bought our XC70 in 2003, the battery for the diesel version was installed. When we
sold the car 13 years later, that same battery was still in good shape!

[BenF]

If the dealer replaced the battery, check to see if the replacement was the correct spec. The one now in may simply be inadequate.

It was, I put in an appropriately sized one yesterday so maybe all of my talk about the alarms was nonsense !

[Reverend]

A large spark when connecting the battery indicates an abnormal load is draining the battery.

Not so. When the ignition is in position II, a lot of things are on, so unless you're really quick with the battery cable it will spark.