[Solved] At a music festival in Germany that takes place at an airfield

Survey whiskers! They're pretty commonly used to mark buried utilities, sightlines, possible expansions, etc. It looks like this one has been gone over with a mower. They last a really long time compared to stakes or poles (in that they can't be easily knocked down), are much faster to install and are often easier to spot in low/medium-low grass and brush than flags or spray marking.