I think you have quoted a post in a thread using the button that is the multi quote button (it is directly to the right of the 'reply' button). Then did not reply in that thread. I think the system then sometimes holds them over to the next time you reply to a thread. It might go away if you log out and then back in unless you want to go back and through every thread you have looked at today. I don't know another way to find out which post is quoted.