SF PAUL

My 2019 E450 coupe's alarm goes off randomly, usually in the garage. The car has a new 12 volt battery and both fobs have new batteries. When the car is in Standby Mode (The alarm is not available it says) and unlocked, it still goes off. I understand if the car is unlocked the alarm can't go off but it does. So while in Standby with the doors unlocked the alarm goes off. My dealer says no error codes and it needs to go off in the dealershiip to trace the source. Even when it's been sitting there 3 days, it doesn't go off? Any ideas? Desperate for help.