Context: the Thin Blue Line is symbolism for the role of police in maintaining public order. It has been co-opted by the alt-right. There is also a Thin Red Line to represent nurses. The French Wikipedia happened to use it in a legend to explain what the red lines on a map mean.
They have used a file named “Thin Red Line for Nurses Flag” as the key for the red line on the map. If you click on the red line in the legend, it will take you to that file.