Chums Zip Pull Whistles
This is simply a good idea. I don't know why more manufacturers don't add them to their kit as a matter of course. The international sign of distress out in the wilds is 5 long, loud whistle blasts so to have a whistle attached to your favourite jacket or rucksack is a no brainer.
They come as a pack of three (yellow, black, grey) and they can attach to a lanyard, key ring, or zip. The cord is reflective so is great for visibility.