I see this message too, Dash. There are two possibilities I have come up with:
1.) It is set to come up around game 13 whether you have any to sign or not.
2.) You may have signed a player(s) to a 1 year contract during the season and you can't sign them again until they go to free agency, but the message comes up because there are players with 1 year left.
For those doing many leagues, I find that putting a sticky note on the homepage for each league and typing a reminder that there are no resigns saves some time.