It is good to carry a whistle with you when you go into the wild. You never know when you might need it to call or alert someone, or even scare some animals.
We always carry one whenever we go out. One for me, one for my hubby. Even if it is just to go outside and walk the dog.... we never know when you might find yourself in a situation that you will need the whistle.
That's a wonderful idea! I never thought about carrying a whistle OR a mirror but it's a wonderful idea to do so. We generally have a "whoop" that we all know that we use when hiking. It's distinctive and can carry quite a long way. I use a variation to call the cats at home to dinner while they are roaming the woods. I was amazed that it carried as far as it did.
This one came to me the other day and I have it added to my core emergency kit that I carry in my day pack. This 1 item eliminated 4 separate items and added a new one (thermometer).
Here is another whistle tip ... carry your whistle either attached to your back pack shoulder straps or on a lanyard around your neck. The reason I say this is because if it is tucked away in your pack and you become incapacitated you may not be able to access it. If it is attached to a shoulder strap or around your neck, you will have ready access to it even from odd angles should you become injured or otherwise unable to get your pack open.
Edit: I have a Fox 40 whistle attached to one of my shoulder straps on both my large pack and my day pack.