Once upon a time, NPC women used distinctive whistles (the kind of whistle one creates and not the "sorority whistles" that come up when one does a google search). This post gives some information on the whistles. It's what I could find using the resources I had at hand. I'd love to add to the post, so please let me know what I am missing.
Sorority whistles may have been the first social media used by National Panhellenic Conference organizations. Imagine being on a campus and hearing your organization's whistle. What a comforting sound it must have been, knowing sisters were close at hand.
Sorority whistles first appeared in the 1880s and by the 1910s, many of the National Panhellenic Conference organizations seem to have adopted a whistle.
The rest of the post is at: http://wp.me/p20I1i-q7