Have you ever seen it block anything on Auto-Decide?
I would wager not, since all it does is scan "suspicious events", and it does not block those, it just logs them as suspicious.
So there is your reason for no alert.....there is nothing blocked, so no alert is given.
As to how to set it....thats up to you. "Ask" is only safe if you know the right answer......auto-decide is passive. I just leave mine alone, and it leaves me alone.