Tonight when they all go to roost, go inside the coop and remove the hen to do an inspection with a headlamp or flashlight. In summer when flies are out, there can be maggot larvae that can be deposited on poopy vents, injuries, or a prolapse. That is flystrike and deadly. More than likely, it is just a soiled vent, but check it out soon. If she is soiled just stand her in a pail of warm soapy water, and rub it off.