OK... when you removed the battery from the car did you do it 15 seconds AFTER removing the key from the ignition? This would have turned off the powersounder for "maintenance".
If not..... I think it's a tech 2 job or at least remove the powersounder (found under the windscreen wiper scuttle bay). If the batteries are duff on the powersounder then this can also make this happen.... replacement batteries are a nightmare to find but if you do a search on the forum there is a supplier that does them... if not it's a Vauxhall part for the full power sounder... last price was around the 50-60 quid mark.... then apparently(not sure on this) it needs Tech 2 to marry it to the car as mentioned above.
The power sounder that I've recently removed with dead batteries goes off as soon as power is applied. Maybe removal of power for a while was the last straw for the internal batteries?
@Kev... I think that's what's happened to mine.... battery went a bit dead after using it to play loud music on a sunny day... recharged the main car battery and the bloody alarm keeps going off randomly.