Someone wired up something to the horn because the OE alarm systems on the w124 and w201 (and others) did not "beep" the horn to alert you that it was armed.

They armed via locking the car from the outside and would go off if the car was opened from the inside (like if someone broke a window and reached in to unlock it).

__________________
Daily Driver: 02 E430 4MATIC
In the family: '03 E500 // '04 ML500 // 64 220SE