personally I have never heard of an OEM alarm having passive arming (i.e. locking doors and arming the alarm by itself after a given preset time) most if not all aftermarket alarms do it, but as far as OEM delayed locking is all I have ever seen OEM wise, and delayed locking requires you to tap the lock button, it is not passive, if you don't touch the button it won't activate the lock.
though I have never tested to see if my vehicle locks itself, it is a given for me, I open the door I tap the lock button and hear the 3 chimes indicating delayed lock is active.
I always wait for the horn toot before walking away.. but I seldom ever use my key fob to lock the doors, 99% of the time it is delayed locking, simply because I usually have my hands full (laptop, lunch bag, etc) and it is easier than fumbling for the fob after I retrieve everything from the trunk.
I'm only responsible for what I say, not for what you fail to understand.