What does a red check mark on the OS X login screen indicate?

This happened to me. I believe this only happens when your computer is asleep and the battery dies completely and you plug it in before you snap it out of the sleep. It is supposed to let you know that that account is the one that you are still logged in on. I still can't explain why you have to log in twice. Strange huh I'm not 100% sure though.

It means that You were not logged off from this account. Perhaps You've made Fast User Switch or directly go to Login Screen without logging off.

I believe that it means that you were still logged on when you went off of your laptop/computer. This has happened to me a few times.