It gets even weirder. A few years ago the New Yorker had a short article talking about SOS, the emergency signal used in Morse Code.

And they wrote it S.O.S. Which is patently wrong.

SOS is not sent in Morse like this, as if it were three separate letters:

dit dit dit (pause) dah dah dah (pause) dit dit dit

(The letter "S" is "dit dit dit" and the letter "O" is "dah dah dah".)

It's sent all at once without pauses between the letters, because it is not the three separate letters at all. SOS is a "prosign", sent like this:

dit dit dit dah dah dah dit dit dit

To help indicate this, SOS is customarily typeset with an overbar connecting the letters, like this:

  ___
  SOS

(If the bar above SOS has gaps in it when rendered on your device, imagine that it is one solid bar.)

It's not an initialism, not three separate letters, and definitely not S.O.S.