I'd bet my left testicle there not ment to have a nut there. There's already a fastner directly under it. It's probably there as part of the manufacturing process or for placing a optional part we don't get. Not sure why you have seen cars with them I've Seen over 20 polos after I first read the missing nut post and not one has had them. Many cars have bolts sticking out without a nut especially around the firewall for the above reasons.