Another problem with any signalling device is that some people don't realise its use as such.

Many years ago i was on Dartmoor in the south of England and heard a whistle blows. It was right on the edge of my hearing so although i could hear it sporadically i could not get a fix on its whereabouts. I stopped and listened. whilst doing so a large walking party that was following me on the same route neared and i asked if they had heard whistle blasts they said no. Just then we all heard the whistle blast and people in the group said oh there it is and all pointed in different directions. I cupped my hands over my ears and determined the correct direction and started walking in that direction, saying i am going to have a look as someone was in trouble. It was only then that some in the group actually recognised the significance of the whistle blasts and a debate started on whether they should carry on on join me in looking for a casualty. the thing that struck me was the group looked quite experienced, had the right kit and everyone of them was carrying a whistle attached to different parts of their clothing but it still did not register with them that whistle blasts was from someone in distress.

Something i have carried and used to get the attention of groups spread out over large areas is a horn, the type you used to see on old cars, a horn with a rubber bulb that you squeeze. Modern ones of these are small, strong and light. I first came aware of them seeing demolition teams use them to warn that they were about to detonate when dropping chimneys and other structures. the cave rescue guys at my caving club used these to communicate in caves too. I must replace my horn as it went walkies.