Mine has been doing this a few times a week for some months now. It didn't do it, not once, for the first few months. I don't think it is me who has changed behavior. And, there have been instances where I am absolutely positive a good 5 or 10 seconds elapsed between engine shut down and key removal, door open and so on.
I don't yet know what will happen if I don't reinsert the key and shut the alarm off, because I've done that every time. I'm afraid the car might have a temper tantrum if I don't, LOL So, what happens if you ignore the beep and exit as usual?