Alarm going off - what alarm? does it come stock?

I have a new 2013 level 3 with solar roof.

Apparently it comes with an alarm. It has gone off in my garage a couple times and I am stuck there looking for a way to turn it off while the horn beeps for what seems like a minute.

The first time it happened I had opened the door and was going to get in the car, then got distracted and walked away for a few minutes and it started going off. I figured maybe it had something to do with that.

The second time, just a few minutes ago, I was seated far away in the house, two stories over the garage, and it just started beeping.

I got out the owner's manual and there is nothing about an alarm in the index, but I googled and found something about hitting any button on the key fob. I was sitting on the keys. Maybe that triggered it?

Question 1) Apparently I have an alarm in the Prius. Is that stock? I did not pay for one as far as I know.

Question 2) What sets the alarm and what turns it off? I am aware of the lock button and the unlock button on the keys and also the sensor on the front handles. Is that also alarming the car? But I am pretty sure that I did not push the lock button on the key or the handle when I was last leaving the car.

 

This evening the key was in my back pocket while I was sitting so it is certainly is possible that there was pressure against the red horn button. But the first time it happened it was in my back pocket and I was standing, so there would be a lot less pressure against the button.

This new car is a major change from my previous vehicle, a 1994 Chevy Astro van, with an AM/FM radio and hand crank windows. Are alarms all so standard that Toyota didn't even bother to mention it in the owner's manual?