Just to confirm, you've spoken about 'the battery', are you referring to the small battery for the alarm, remember the power sounder doesn't get its power directly from the main under-bonnet battery, it only trickle-charges a smaller battery inside the offside wing for the sounder.
Changing the power sounder would do nothing, certainly changing it twice even less!
When the battery gets old (often something like about 10-odd years old) it doesnt charge as well as it used to. When the sounder senses reduced power it reads this as a wire's been cut ie: someone trying to break into me, thus the alarm sounds.
As a cure many remove sounder, best to remove the battery too, as they are known to cause fires (!) Though I personally would favour replacing the battery with another suitable.
It's a common fault, and the drain from using various fans etc. would be consistent with a poor/old battery, causing the sounder to go off. If Vauxhall didnt make you aware of this and just sold you a new sounder, thats just an example of so-called experts being thick. If you know all I've just typed already and I've answered a question you never asked, well that's just an example of me being thick!