The mesh of a mist net isn't made of single threads, but of very fine threads sort of twisted together, and there would be delicate knots every few cms securing the vertical and horizontal threads at the intersections. Here's a pic from one of our banding sessions this year:
It's illegal to leave a mist net up unattended and they're pretty expensive so they're usually not abandoned. If what you're seeing are just threads with no knots, then it's likely not a mist net. If it matches the material in that pic, though, I hope you can find out who's left one out, because it's a death trap to smaller birds!!