I know there is some long bolts in the frame to Rad/core support but I don't recall them being like that.
Honestly it looks like an aftermarket bolt one because of the raised markings on the head and two because it isn't rusted (seriously speaking most body bolts will have surface rust at least) so probably just a stray that you picked up.
If I may offer you some comfort, I had one of those bolts the other day and can no longer find it although I didn't need it thought it was strange that it was gone. Perhaps it ended up in your truck somehow the other day when you were here. I was using it for my rack and pinion project.
mike i think it is more amazing you know something is missing hahahaha. well ill bring it over next time i come. i really dont think its from my truck. mike just playin jokes on me leavin random bolt under my engine bay cause i send all his email contacts porn haha jk