You dont think that if Steven walked away he is living as Steven Koecher do you? Although I have seen it happen before, that would take a lot of nerve.
My guess would be that he assumed another identity if he stayed in Las Vegas. When I was pondering the possibility of that, I wondered where he could be that he had not really stood out in the last 5 months. And thats when I started to consider him hanging around a college environment.
I understand, but he'd need a support system and an income, or be VERY attractive to someone who wants to support him 100%.
As for identity: he could live undetected, IMO, under any identity he chose. He's an average-looking guy in every respect. He's written (as I mentioned here several times) under the pseudonym "Steven Thell" and could use that as a job reference.
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Not having a complete list of close friends puts me at a disavantage for answering your last question. Since you are the family contact, and you make it clear you feel well versed in this case perhaps you are better able to answer that one.
Remember the old joke about how to tell a good friend: "A good friend will help you move".
If someone helped him move, which would have been the purpose of all that driving on Dec 12th and 13th, then it shouldn't be hard to find someone who was also gone during the time period in question. Among all of his known friends, there's no one that fits that description. Among his non-friends, their whereabouts are known and in some cases, self-admitted.
If there's an unknown friend, then we're back to the "how did they link up" issue.