How should passengers report SOS signal sightings? On January 23, 2015, I flew from Buenos Aires, Argentina (SAEZ) to Santiago, Chile (SCEL). It was a very comfortable night flight in a Boeing 787 Dreamliner of LAN airlines, though it was delayed.

During the descent and a pronounced turn to the left at approximately 4,000ft above the Andes Mountains, I saw through the window a wood fire in the middle of darkness with some shadows around. Then a brilliant white light blinking: three short flashes, three long flashes, three short flashes. I thought: Wow! That antenna blinked like an SOS signal!

One second after the first sequence, again: three short, three long, three short. Then I unfastened the security belt to run and tell the closest flight attendant: Sir! There is an SOS signal there, and he started to give attention to it.

After a successful landing on runway 17 L/R, I asked the flight attendant if they have a procedure to handle this kind of sightings. He told me there is no such procedure and that I should go to inform the civil protection office.

Due to a lack of enough time before the next flight, I asked a security officer to inform the civil protection office, giving her the flight number, exact time of the sighting and after calling the person in charge, she asked me for my name and details about the sighting. Trying to be descriptive, I clearly told her all details before running to catch the next flight and almost losing it.

How should passengers report SOS signal sightings?

P.S. The place of sighting was somewhere over the mountains in the west of Los Andes in Chile.
There is a procedure, at least for the aircraft crew.

It is appropriate for a flight crew to transmit a "pan pan" or even a "mayday relay" so a passenger should immediately inform the senior cabin crew member, usually by asking to  speak to the "purser".  SOS is an internationally recognised distress signal.  If the purser did not act upon it, I would politely, but assertively request that they inform the captain since these procedures are part of standard pilot training and flight crews are required to respond to potentially life threatening situations.

Reporting emergencies or urgent situations is not restricted to the aircraft itself.  It can, and should be used, where any threat to life or safety  is real or possible.  ATC in turn can contact the appropriate local resources depending on the situation.

An example would be if I was flying and witnessed a fire or perhaps a road crash and emergency services were not already in attendance, I would not hesitate 
to contact ATC and be prepared to offer assistance if possible, for example relaying position or exact circumstances.

You did the right thing.  Perhaps you could have been a bit more assertive at the time, when the cabin crew member incorrectly told you that there is no procedure, since minutes can count.