I carry a bunch of whistles.

I have 3 large key rings (work, home and play) and each have a whistle of a different colour so I can quickly tell one from another. I am now switching brands of whistles on the key rings (they were all Fox 40's) so I can tell by feel which ring of keys I have. I carry another whistle on my work belt, actually 2 if you count the 1 in my mini-PSK. When off playing in the bush I have one clipped to my Nite-ize pockets pouch and another buried in the mini-kit in my EDC cell phone pouch. More whistles can also be found in every vehicle, boat, ATV, snowmobile, PSK and FAK plus in my shaving kit attached to a small flashlight. Whistles are a cheap but very effective signaling device.

I even have a whistle collection that I use when teaching signaling to children, the kids blow the whistles and we rank them by loudness.

Mike