I can't help thinking that if my adult son said he was going out to engage in unsafe (possibly) behavior, I'd strongly urge him not to do so. And, if his last words to me were, "Goodbye, I'm never coming back" that it would be really hard to report him missing.
Also, my brother is totally estranged from his kids and they've totally turned their back on his whole side of the family, including me. I can see his son running off, his mom saying, "Whatever, he's an adult, he'll be back when he wants money" and him not being reported missing.
There are all kinds of reasons for an adult never being reported missing, it doesn't mean they weren't loved.
This whole thing is happy and sad at the same time.