You can't leave it in there. I would try fishing for it with some wire hooked on the end. You might go to an automotive parts store that loans or rents tools and see if they have a borescope, or a very small inspection mirror might go in. Unfortunately, I believe the mesh is stainless steel, so a magnet won't do you much good. I don't think you are the first here to loose the filter. Maybe they will speak up with how they got theirs out.