How should passengers report SOS signal sightings?
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.