I also think it's aftermarket alarm related, considering the no-start / hard reset. The horn going off may have been an alert to an alarm problem, possible tamper or intrusion/ zone alarm. Or if you still have a stock battery, it may be dying, as most stock batteries have been replaced, I think. Maybe a stock battery poll is in order.

As far as replacing the stock alarm goes, I think those who have replaced it is due to very limited coverage... no rear door , glass break, impact or jacking coverage. I'm not sure that it even locks out the starter if you were to enter through the tailgate or a broken window.