My old top had the same problem (good my window collapsed and my insurance bought me a spanking new top), but again, my old top had that line and could never figure out what it was. It's not you or a cat, or something random. It gets worse as you make the top go up and down. I had it for about 2 years and slightly kept getting bigger, but didn't appear to rip. But the cause? It spooked me. Could be the fabric sucking in that area.