I wonder if he signed the Eagle Peak register.
That route up from Eagle Pass (where you veer off the trail and start to ascend to the peak) looks very dangerous. It's easy to see how Austin could have had an accident or gotten lost. There doesn't appear to be a clear route up/down, so it must be very difficult for searchers, as Austin could be anywhere. And since he didn't seem to make it back to his camp at SD8, this seems to be the most likely place he ran into trouble.
Here is a link to a YT video of 2 guys climbing to the peak. They came to Eagle Pass from the trail to the north, while Austin came at it from the south. But they all likely started their ascent at Eagle Pass. Go to 11:00 in the video to see that point to get a sense of the ascent.
It's also possible that he ran into a bear on the trail once he descended back to the trail, but I would presume there would be evidence of a bear attack, wouldn't there?
As an aside, I'm confused about the comment that his truck is missing. He got dropped off by boat and was supposed to catch a boat back, so he wouldn't have had his truck with him. Is it missing from where he parked and set off by boat? That wouldn't explain why he didn't make it back to his campsite though.