From memory, as I have no car now, under the drivers door handle is a key hole. You might find this activates the alarm siren. I think there was a key slot in the door visible below the locking mech as the door opens to switch off the alarm.
My battery failure was caused by water getting in the radiator fan relays just behind the top of the radiator, switching the fan on. Yours may be compatible with Vectra C, but my 2.4 needed one from italy.
Keith