The only thing I would do is use a search tool (find command or GUI util) to search for it, but I bet you are one step ahead of me on that one too.
If the file is in cache, then it probably has a funky name anyway and this won't help. I would download a file, denote the size, and do a search for files with that size. The find command has a -size option that will do this.
After this, I am at a loss. Hope you find your answer. Please post when you do. I would really like to know that is happening.