At this pint its a bug.
Car alarms will go off forever every car has a chance even one you have the key to. I have a feeling its realted to another bug where i do but i dont have the key to emergency vehicles that spawn. I can walk up to them and the key icon flashes but it says locked, and i definetly dont have a key, but the UI says I do.
Even mods that let you claim vehicles wont stop the alarm from going off. 2 weeks later and i have the key but if i bump a post or randomly driving the alarm starts to got off.