I think it's a setting in the CCF.
I've checked the "as built" ccf, under safety/security and locking, and "vehicle horn on alarm enable" is set to yes.
It says 63% of cars were set to yes from factory.
If that is true, I'm guessing a lot of dealers turned it off when new.
I can't currently alter the CCF to test.
Also my post title is incorrect, the horn beeps once on lock, twice on unlock.

