Hi @Brian143 @Airbnb's verification process only confirms that a guest has submitted an ID and that's it. It's all done by bots, there is no review process or background check in any way.
This is a crappy thing that happened to you, and you dealt with it in a smart way, refusing the other guests entry and staying up all night.
I would do the following now:
1) Change the locks wherever the questioned guest had access. Save the receipts but don't expect that Airbnb will cover you the cost. But for your own security, do it right away. It's a legitimate business expense that you can submit as your taxes, but most importantly it's for your own sense of security.
2) Don't confront or have any discussion with guest about what you've found. Simply ride out the booking until he checks out in the morning, collect the key and thank him. Don't get into convo's about his behaviour last night trying to sneak in unless he brings it up, then just say "yup, house rules". You are trying to not give away anything, and if he's a con, he'll look to rile you up.
3) Contact Airbnb, tell your side of the story and ask what your recourse is. I don't think you have any (you should, but they'll argue he didn't actually do anything other than try to break house rules).
4) Leave an accurate review and give him a thumbs down on review, indicating you do no recommend. That way, he will be unable to use Instant Book in future. In area where oyu give feedback to Airbnb (and don't mistakenly use the "feedback to guest" box), tell Airbnb that you were not comfortable with this guest and that you found he had various records and that you do not think he should be allowed to use Airbnb. Try to have him banned.
Is the guests name Francis, or something along those lines? It's similar enough to a situation that I had last year -someone that I was sure was a con, booked and then tried to sneak in all sorts of people, was surprised that I live downstairs so I was onsite monitoring.