All good thoughts - thanks. As for jokesters, the running prank with the crew I was riding with was turning on grip heaters in summer. Plus, no one had access to the trailer or bike during the move, so I am still convinced something vibrated loose and fell to the trailer floor. I do have crash bars, I have not upgraded or replaced the stock caliper bolts.
I do agree that just because the nut fits the bolt, it does not necessarily mean they go together. It is possible that the nut vibrated off of some other bolt and this bolt vibrated loose from something else. But I am still inclined to think they do go together.
Hopefully, someone recognizes where this style of bolt is used. But like Boomer, I did not think it was used anywhere else besides the calipers. I'll keep looking. Thanks for the ideas.
Rob