Anti-Theft Alarm System for VW Golf III 1994

Hi, my brother has a Volkswagen Golf II from 1994, it's a TDI with 90 horsepower and a GT trim.
Recently, the alarm system has been malfunctioning.

The problem is that once he locks the car, after a few hours, she can simply open it. When he then goes to the car, the indicator light doesn't flash anymore, and he knows something is wrong. However, as soon as he unlocks the car, the system deactivates, and the engine starts as if nothing happened.

"He's been having this problem since Saturday. He left the car with a colleague, and the alarm went off two or three times there. Yesterday, it went off five times without anyone doing anything. Nothing was stolen all night, and then it started going off again this morning."

Does anyone know what might be causing this and how to best fix the problem?
Or has anyone else experienced a similar problem?

First of all, thank you.

Normally, the error log stores the reason (which switch triggered it) for the last alarm.

Okay, I'll go ahead and read that then.