The SOL Emergency Bivvy with a rescue whistle built into the stuff-sack drawcord.

Staying warm and being found are the two halves of an unplanned night out, and they are easy to solve separately and then forget one. Putting the whistle on the bag means they arrive as one item and leave the house as one item.

Heat-reflective lining returns your own body warmth. Packs to about the size of a soda can.

Worth the few dollars over the plain bivvy for that reason alone — it removes a decision you would otherwise have to remember to make.