Why does my alarm go off when I'm right next to my car?

My '05 Prius has a tendency to start blaring the alarm, even when I'm in
it or standing nearby. I walk away and off it goes.

I have the key in my pocket (SE/SS system) and the last time it did it,
a few months ago, I replaced the battery in my remote and the problem
went away, or so I thought.
 
It doesn't do it always, maybe when I'm in certain parts of town. What
could cause such a thing?

FYI, I came home, locked it and so far, blessed silence.

--LG