Simple solutions often turn out to be the best. A small RF transmitter, powered by a hearing aid battery and using a piece of guitar string as an antenna, is used to track monarch butterfly migrations over thousands of miles of travel. Anyone can tap into the signal if you tune to the right frequency.
Excellent anecdote! Interestingly I've experienced the story told in a slightly different way. Another reason strangers are more likely to gather at the exclamation of 'fire' is for the sensationalism of it. For the same reason people slow down on the highway as they pass an accident on the other side of the median, people will rush to see a fire. They want to see something exceptional from their everyday routine; they want to be entertained. While this notion is somewhat distasteful, and far less altruistic than we might hope, it is nevertheless a powerful and effective tool for generating an audience. If we think about it, I believe we can all agree that the general media sensationalizes events regularly.
This success in being able to draw a crowd is why the technique is suggested in self-defense. Often when atrocious acts become public, the perpetrators are discouraged from continuing. Taking this one step further, if enough people crowd around a burning building, a special few might just run in to pull someone out.