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Hi, welcome to Just Answer. My name is***** it is possible that water has found it's way into one of the control modules on the car. Leaving the car unlocked may keep the alarm from going off. The alarm is activated by the rear signal acquisition module. The module is located in the cargo area beneath the fuse box. It is very common for water to find it's way into the module. That is where I would expect to find the problem.
Let me know if you have any questions.
I've attached a picture of the module location below.