We/I *usually* (tho not always- its heavy) throw into someone's pack an old trusty friend, my Icom A4 airband handheld...
If I was ever inclined to carry something out of band "just in case", that's exactly what I would be carrying. Just listening to 121.5 and hearing all the captain's welcome announcements when he picks up the wrong microphone, and all the catcalls of other pilots chiding him for being "on guard", gives me the impression that there are a helluva lot of ears listening to that frequency up there.
They're listening specifically for someone having an emergency, and they all know exactly what to do if they here something. Unlike a 911 dispatch center who's first assumption is going to be hoax, or ambush.