Welcome, I am Chris (aka-Moose)
The car while off and locked is monitoring the door ajar, hood ajar, trunk ajar, and battery voltage. When it sees a power fluctuation or any of these things opened. It thinks a theft is occurring which is why the alarm is going off. The unfortunate part there is not one way to diagnose it. You have to try things and retest. The first suggestion is to use spray lube like liquid wrench and spray lube the door ajar switches through the door latch to see if one is partially sticking.
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