The traditional version of that is a bright orange square or signal mirror. They pack down really small and are extremely visible. Similarly little IR and retroreflective glint squares are handy for spotting people at night.

That said if you're doing anything serious away from people carrying a GPS and EPIRB/SPOT is way more effective. The latter working even in the most remote locations. It's no good being able to signal if no one is even looking for you.