This sign was put up sometime this Summer. I think it's cute. We pass over a one lane bridge on our way to the lake, this is right beside it. Lots of fox in the area but I've never seen one on this nature made bridge. Maybe the sign is there to encourage them to use it. :)
Research fox behavior and see if they are more active at a particular time of the day and stake out a place along the trail and maybe you'll catch one.