vectra est 55 plate car alarm?

hi there, we had 'Safeguard' message on screen and took to vauxhall dealer who said it would cost £500 to replace both boot side windows. managed to get my brother (ex vauxhall now vw mechanic) to fix for us and clear message etc etc. All fine for at least 5 times locking,then drove 60 miles back home and alarm going off every 5 mins or so.
Okay for a day or two then plays up again. the other afternoon it had been parked and locked for 2 hrs till the alarm went off then it was every 15 mins so had to leave it unlocked. any ideas as we are baffled, perhaps car doesnt like our driveway anymore!