If I remember correctly from my flying days, it is a general warning
from a boat or aircraft but there is no immediate danger to life. An
example would be to warn of something that could be pose danger, rather
than had happened. Mayday would be the next 'level', when the muck hits
the fan and help is needed. (A bit OTT for calling an ambulance etc) I
don't recall pan x 3 ever being taught in the RAE etc.
There is also Securite x 3, more for nautical stuff, and (in broad
terms), like pan x 3
As for the incident, in the days I referred to, that did happen- more on
repeaters, especially in London.
These days, people tend to reach for there mobile phones but, I would
like to think, people would (in the main) react correctly if they heard
such a call.