Well, no one has posted here in a year, but I came online tonight looking for this exact thing. My brother's acoustic guitar, propped against a couch in our living room, has begun to "strum itself" this week. It's a dissonant, bright sound, not a chord, involving many strings, and it's quite loud, but it's fairly short.
I have no idea what could be causing this and it's freaking me out, too! As the OP said, it tends to happen when it's quiet, and it frequently happens at night, but not always. I don't know about any temperature changes going on - it's never happened in front of anyone else (darn it!!), and always when I am sitting in here just reading, not even walking around. We have no pets, no open windows, no air vents near the guitar, no drafts, no curtains near it that could be brushing up against it, nothing. Tonight, I happened to be looking at the guitar when it "went off," and I could see the strings moving but couldn't tell what had moved them.
We have a piano and many other guitars, even in this room, and it's just the one guitar that does it. If anyone sees this thread, could you please explain how this phenomenon works again? I don't really get it and I am about two more serenades away from throwing the thing in the attic!