This article (including video) credits a signal mirror for the rescue of New Zealand woman Kushila Stein adrift in the Aegean sea north of Crete this Sunday:

"Fortunately, by using a mirror to reflect light, Kushila was spotted by the Greek Coastguard and was saved after two nights adrift."

Other articles cite the signal mirror was used, but not what the rescuers actually spotted:

The New Zealand Herald said: "She also pointed the small mirror she kept in her backpack at the sun to try and attract the attention of any planes that flew over." and "She had been located 55 nautical miles off the north of Crete about midday on Sunday."
The BBC said: "Ms Stein put a red bag on her head and used a mirror to attract attention, reports say."
Yet another report said: "Whenever a plane flew overhead, she pointed a mirror toward the sun, hoping to create a flash that would attract passengers' attention. "

The Daily Mail Australia article didn't even mention the mirror.
A signal mirror should backup a radio distress signal, like a 406 MHz PLB (ACR PLB) (Ocean Signal PLB)