1997 Lesabre alarm goes off


My daughter has a 1997 Buick Lesabre Limited with a factory security system and the alarm goes off for no reason usually in the middle of the night but not always. I took it to my mechanic and he couldn’t figure it out and I took it to a mechanic at the local Buick dealership and he can’t figure it out I also looked on line and found a ton of recomendations, tried some but have had no luck. Any suggestions would be greatly appreciated.