In the first instance you should have the vehicle checked on a diagnostic tool such as Mercedes STAR to see if any fault codes are stored and what the most recent alarm triggers have been.
The most common problem we see is with the alarm siren - this has its own tamper circuit inside & when it fails the alarm will be triggered. This will usually have fault codes stored or alarm triggers with the siren to confirm this.
If faulty the siren should be replaced.
Best regards, Paul