That mysterious bolt seems to pop up in threads every so often. It seems it's a packing bolt from the crate that the bike is shipped in and somehow gets lodged into the bike when it's uncrated at the dealer, is my best guess. A bolt that large would be quite obvious if it were missing on the bike.
In any case, no one has crashed (that I've heard about) from it missing on their bike, so I think you're safe.
