Years ago on Answerbag I had a friend with whom I chatted quite frequently. Always looked forward to our encounters. Then she was gone. No goodbye. Just gone. I figgered she had just moved on and was disappointed that she didn't say "so long Rosie". A few weeks later I got an email telling me she passed away! I am still so sad about that all these years later.
Apparently the sender of the email was thoughtful enough and kind enough to notify everyone on her Answerbag friend's list of her passing. Even in those days I asked a lot of questions and couldn't keep up with my responses so I was ALWAYS BEHIND. From time to time then, as I do now, I'd respond very late. So I did that and found many of her responses I had not answered. Which increased my sadness. So I did. Every one. I apologized to her (hopefully in HEAVEN she knew how sorry I was) and simply replied as if she were still alive. It haunts me still. So when people disappear from your life without notice maybe it's because they didn't want to but circumstances took them away.