Funny you'd mention this... just two days ago I lost a hairpin on a 2" receiver locking pin going through thick brush. That didn't surprise me. However, I have a rear hitch with a hitch pin, that has a linchpin keeper. I lost that linchpin. Don't know why. Must have snagged on a vine or something.
I keep about half a dozen in my trailer box just because of how often they come up missing. I'd like to find something more secure as well, but don't want to go the bolt and nylon nut route just yet, but its coming to that.