I was wrong in my previous comment. The 3 dots is called an ellipsis.
The iMessage ellipsis begins when the first character is typed. If the sender stops typing but does not back up to delete the showing characters, the ellipsis will continue for 60 seconds. Then it stops and there is no message. If we are messaging, and I type one character, then close my messages app and go to Facebook, you would be left watching the ellipsis for 60 seconds or until it stops. Then you would wait a moment more for a message to appear. But it would not. If I were to type a word, then back up to where no character shows, then the ellipsis stops. And again, there's no message. As to what the sender is doing is a guess. They may start a message, then stop and move on to something else. They may accidentally hit a letter, then close the app, leaving you watching the ellipsis for 60 seconds. The ellipsis is not an indication that a message is incoming. It just means at least1 character was typed.
It's not an iOS 8 bug. It's been going on since the iMessage ellipsis was first implemented.
Sent from my ancient but trustworthy iPhone 5