How should passengers report SOS signal sightings?
You did it right, the head of the cabin crew must inform the captain, as you asked for, the captain must inform ATC and ATC the rescue organization when the information is credible. These matters are highly subject to judgment at any stage of the reporting chain, and it would be interesting to have a view from a legal person about precedents.
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.
That's probably wrong, the attendant should report this to the captain. Not doing that is probably a professional misconduct. As for the informed captain not complying, they would be exposed to legal action if it was an actual distress situation (see below).
In addition to this crew obligation, anybody can be, to a variable extent depending on the jurisdiction, liable to a duty to rescue. An SOS signal is possibly a sign of distress, it triggers this duty to rescue.
1. ICAO Chicago Convention
The ICAO Chicago Convention is an international treaty, a law for the signatories, implemented in the State legal system by the aeronautical information publication.
The Convention includes Annex 12, dealing with Search And Rescue and its article 5.7 providing a guidance for managing the interception of distress signals. (In aviation terms, the pilot-in-command is the person among the flight crew, legally responsible for the flight, like the captain of a ship):
5.7 Procedures for a pilot-in-command intercepting a distress transmission
Whenever a distress transmission is intercepted by a pilot-in-command
of an aircraft, the pilot shall, if feasible:
a) acknowledge the distress transmission;
b) record the position of the craft in distress if given;
c) take a bearing on the transmission;
d) inform the appropriate rescue coordination centre or air traffic
services unit of the distress transmission, giving all available
e) at the pilot’s discretion, while awaiting instructions, proceed to
the position given in the transmission.
It should be noted this article is connoted to rescuing ships/aircraft, which was the concern at the time it was written, but is expressed in such a broad way it becomes applicable to any distress message.
2. Example: Implementation in the French Air Regulations
In the French AIP for La Réunion and Mayotte Islands, those articles are relevant:
GEN 184.108.40.206: Rules to be observed by aircraft commanders
1) Pilots who witness a situation in which people are endangered:
When an aircraft commander of an aircraft witnesses a situation in which
people are endangered, he shall proceed as indicated below unless it
is impossible to do so or, based on the prevailing circumstances, he
considers it is unnecessary to do so: [...] notify or have someone
notify, as appropriate:
- the area control center (ACC), which shall in its turn notify the ARSC [...]
2) Informed of a:
a) distress message or signal When an
aircraft commander intercepts a distress message or signal, or is
informed by any means, he must proceed as follows in compliance with
applicable telecommunications procedures [...]
b) request for
assistance by message or visual signal: When an aircraft commander
intercepts a request for assistance by message or by a visual signal,
or learns of such a request from any source,he shall notify the
appropriate control or search and rescue unit as soon as possible
3. Civil and/or criminal laws
At the State level (for an aircraft this is the country of registration), there are civil and/or criminal laws related to assistance: If someone asks for assistance when in danger, and a person denies it without a good reason, the latter is liable of failure "to assist a person in danger" or to the "duty to rescue". The idea is someone in great distress has a right to be rescued which automatically give others obligations to not abandon them.
In many countries these obligations are relevant of civil code, meaning one can be required to pay damages, but there is no criminal law associated. In some countries, this is a criminal law, e.g. in France not trying to help someone in imminent danger, and when this is possible without endangering oneself, can be punished of a five-year prison sentence. This would apply to any person, witnesses, cabin crew and cockpit crew, ATC, etc.