I'm not sure what goes on with these drug tests. My son was using hard core drugs (he admitted it to me afterwards) but when I had him admitted to the hospital the next morning, after I found him, his tox screen came up showing only Mairjuana. My point is, for some reason these drug tests don't seem to be terribly accurate.
Even if he wasn't using drugs during his disappearance.. does it matter? Is it acceptable for you to be in a relationship with someone who is going to disappear for days at a time and leave you worrying about him? Would you ever do that to him? The reason is irrelevant. In a relationship each partner has a certain amount of responsibility to each other and, in my book anyway, letting the other person know you're going to be MIA for a few days would definitely be an expectation.
My son is my A. I can't describe the amount of love I have for him. However, I've come to realize (only after being worn down to the point that I was physically and emotionally broken) that no amount of love can save him and until HE decides he's had enough, there's nothing that will stop him. Trust me when I tell you I completely understand you wanting to support and love your A but it is 100% true that the best -- and really the ONLY -- way you can do that is by Letting Go and allowing him to figure this out on his own.
Imagine you had a daughter (or a best friend) who was in your exact position.. what advice would you give her?
I know this advice can be hard to take. It's not what we want to hear. We want to hear that if we just do X, our A will miraculously get better, revert back to the person we they were before their addiction, and we'll all live happily ever after. It's not fair, it's not easy.. but, unfortunately, it is what it is.
In your original post you pointed out many things you did not know about this man and said, "who the **** is this man". That is a HUGE red flag that you two are not ready for marriage. As others have said.. stay and support him if you must, but PLEASE, do not marry him right now. Get to some Al-Anon/Nar-Anon meetings to better learn from others who have been there and to learn what it really means to "support" an addict (because it really goes against our natural instincts).
Best of luck to you.