간밤에 잘 주무셨어요
I was chatting with a friend and narrated a sudden event in which an ex-colleague who had seemed all normal but passed away without any signs. She explained that korean people often ask like that: 간밤에 잘 주무셨어요. Literally did you sleep well last night?

Just curious how it relates to my story? What is the story behind such a question?

Question

간밤에 잘 주무셨어요
I was chatting with a friend and narrated a sudden event in which an ex-colleague who had seemed all normal but passed away without any signs.  She explained that korean people often ask like that: 간밤에 잘 주무셨어요.  Literally did you sleep well last night?

Just curious how it relates to my story?  What is the story behind such a question?

Answer

간밤에 잘 주무셨어요? Literally means "did you sleep well last night?"

It is a typical morning greeting for elders. You can accept it as "Good morning, How are you?"

Answer

'간밤에 잘 주무셨어요?' is just a greeting. It has nothing to do with your story. I guess that your friend had misunderstood something.