Beeping if opening and closing door more than once after car is off. Drains battery.

I have a weird condition with my 2009 Prius that will drain the battery in less than an hour. If park the car and turn it off, then open the door, then close the door, then open the door to leave once more, it will beep at me like I left the keys in it. The only way I've found to stop the beeping is to turn it on again, put it in gear, drive a little, then turn it off and exit the car just once.

If I don't manage to turn the beeping off, it's almost guaranteed to have a dead battery when I come cackle and turn it on later. Only the starter battery, though. The main battery usually seems fine.

I'm probably due for a new 12V, but I want to figure out this beeping issue so I don't end up stranded in an emergency.