My baby was violated - almost!

FYI I have a 2006 prius with the VIP car alarm option that I picked up less than 2 months ago.

The past week, I was taking a road trip, and we stopped at Salt Lake City to get some sleep after a long drive. We were pretty much knocked out and sleeping soundly.

The next morning, I asked my fiance to grab something I left in the car, and he came running back in the room telling me that: 1) the smart key doesn't work 2) the car is making an odd noise.

We ran back to the car, and when I listened carefully, the car was making this very soft noise that sounded like what's left of a gasping honking noise. We entered the car using the mechanical key and the power button wouldn't work. I tried turning the light on in the car, and the light was really dim which indicated that the car battery was probalby drained.

So I called the roadside assitance and a dude came and jump started the car. Once he hooked up his battery to mine, my prius came back to life with the alarm fully on, so we turned off that alarm. The hybrid battery was still over 50% charged.

From this, I must conclude that something must happened the night before that triggered the car alarm, and for some reason it kept going on until the car battery was drained. So my questions:

1) Do you think there was an attempted theft? The car didn't have a single dent on it - the glasses weren't broken either.
2) If the car alarm goes off due to an attempted theft, shouldn't the alarm turn off within minutes when everything is safe again?
3) Do you think there was some sort of electrical error/failure that must have set off the alarm? Should I take the car to the dealership to get stuff checked out?
4) Since the car battery was drained once, do you recommend that I purchase a new car battery?