Well, I am that rude idiot.
It was my running watch that I tucked in my bag so I could keep an eye on the time when I taught my lesson (I was substituting for the Relief Society teacher). The alarm went off (for reasons still unknown to me) and kept going for several minutes. I reach down and pull the watch out of my bag so I can turn it off. Unfortunately, the bag was muffling the sound quite a bit. The beep!beep!beep! was extremely loud now and causing several heads to turn in my direction.
Finally, I was able to figure out how to shut the stupid thing off, but not before drawing a significant amount of attention.
Lesson learned: Don't judge another man's beep until you've walked a mile with his stop watch.