An SOS is an old Earth distress signal. Morse code for SOS "···­­­ −−− ···".

The system continued to be used in some form into the 24th century, in 2370 Captain Dawson of the USS Puyallup instructed his crew to send an SOS to Deep Space 9 when his ship came under attack. (DS9 novel: Devil in the Sky)

It is quite possible SOS was being used as a generic term for a more contemporary distress call system used in the 24th century.