car oem alarm on 1998 Buick LeSabre

car oem alarm started going off for no reason

There is a technical service bulletin from GM regarding this issue, and it may be the cause of your problem, but it will take a diagnosis of the system to know for sure. Take this info to a repair shop, explain the problem and the symptoms as best as possible, and inform them of the TSB# 80-90-08: info on theft deterrent alarm system
Info from Bulletin(TSB):
Subject: Information on theft deterrent alarm system
Intermittent activation and/or security light stays on
models: 1992-98 buick lesabre
On vehicles that have forced entry theft deterrent systems, a common condition has
the security light staying on for one minute or longer, or the alarm going off after
the car is locked. this may be caused by the tamper switches or wiring from theft
deterrent/rfa module to the switches.
the car wont start