If it was a paint hook it'd have paint on it, rather than the wear marks where it pulled the upholstery together. I can't tell exactly where it came from as I haven't had my back seat out, but have seen plenty of these in various shapes and sizes while in the auto restoration biz. Sometimes an extra or two will get popped on at the plant, so it's probably non-critical. Just stick it in a zip-lock bag and put it in your glovebox or VX file and remember to take a peek next time you go on a disassembly spree.