The thing that resides in my daysac, that thankfully has never been used, is my emergency strobe light, its been there for a number of years, unused.
I bought it several years ago, in Joe Browns in Capel Curig, on their bargain basement stand.
It still cost me £14, but its one of those items that heaven forbid if you did come to grief, and twisted your ankle or even worse, in the upper reaches of the Carneddau, the mountain rescue lads, would see you straight away.
Its supposed to have a range of nearly 5miles, and the flashing light it gives off, is mighty impressive, far better than a silly whistle.
This thing is fully waterproof, and can be seen from a huge distance.