On May 25, 2013, 5 people whose 19-ft boat had been adrift for 7 days in the Pacific were rescued after a US Navy C-130 search plane from VR-62 ("Nomads") spotted their signal mirror flash at 5 mile range after 3 hours of searching. News reports here:

US Navy Press Release
Photo Album of Rescue from VR-62 Facebook site
Photo of drifting boat, possible weak flash
US Coast Guard news release

US Coast Guard search doctrine assumes a detection range of 5 nautical miles for a sun signal mirror, consistent
with this incident (US COMDTINST M16130.2F , Table H-20, page H-46, January 2013).

Most nations require all oceangoing vessels to carry sun signal mirrors for the purpose of signalling
to rescue plans and ships when in distress:

International Treaty: Safety of Life at Sea (SOLAS) Regulation 11: Equipment of Lifeboats :(xix) a daylight signalling mirror of an approved type
1993 Torremolinos Protocol and Torremolinos International Convention for the Safety of Fishing Vessels (page 80) ; (page 89) ; (page 300) ; (page 309) "One daylight signalling mirror with instructions for its use for signalling to ships and aircraft"
US Code Title 46 CFR 160.151-21 Equipment required for SOLAS A and SOLAS B inflatable liferafts. o) Signalling mirror (Regulation -III/ Each signalling mirror described by Regulation III/ must be approved by the Commandant under approval series 160.020.
US Coast Guard Approval Category 160.020 Emergency Signalling Mirror
SOLAS (Safey of Life at Sea) Requirements for Lifeboats "Daylight signalling mirror with instructions" (Annex 4, Row 19; Annex 6, Row 21;
United Kingdom Lifeboat Requirements 10.1.19 Daylight signalling mirror
FAA Requirement Part 135 Section 167 For extended overwater operations: requires liferaft including one signaling mirror (and lots of other equipment)

Edited by rafowell (05/30/13 10:03 PM)
A signal mirror should backup a radio distress signal, like a 406 MHz PLB (ACR PLB) (Ocean Signal PLB)