I hope you're joking. It's literally SOS in morse code, and it's pretty common hearing the sound in movies / series.
Why? There, by definition, is always a moment when you learn something. You're not born knowing SOS. At some point in your life you learn of it. To reply to someone who expresses that that this was such a moment with "I hope you're joking" comes off as extremely condescending.
I'm not trying to be harsh or anything, it's just that I can't imagine someone not knowing such a basic thing like that - similar to not knowing what the number for the emergency services in your country is.
Never mind, sorry for the bad comment.