I forget where i read the following story, but it impressed on me that a whistle is a worthwhile emergency tool.

The story was from an SAR volunteer. He was participating in a search for a missing hiker. The terrain had numerous boulders. As he passed one of these he heard a wheezy, low whistling sound, so he left his path to investigate. The missing hiker was tucked out of sight a short distance away, asleep, with a whistle in his mouth. Without the whistle, he would not have been found at that time, and most likely that sector would not have been revisited. Now I always double check that I have a whistle with me.