I have never used it, but I remember in the early 90's finding a long thin string (thinner than floss) in the woods during archery season. So someone must have connected in the area I was hunting. I never followed it to the end, but it went on for quite a ways.
Maybe they should bring this back in to popular use and stir the pot like the lighted knock controversy.
You may have found a survey striing from a DNR or FS stringbox. They used them to measure out long runs and were very fine string. I think they may still use them as I thought I saw soome near a guy up off Hwy 2 a couple weeks ago. The string in the Spider Tracker was almost invisible, unless you were looking for it, like a spiderweb....