There is no way to know what our kids are really doing. I thought my daughter was only doing pot. After she got clean, she told us all about her drug use which included speed, downers, psychodelics and even prescription and OTC drugs. I was shocked. She even tried heroin (it was the first time I knew you could try it and not be hooked right away). She did not steal from stores, but had no problem taking money out of my purse in those days...just a bit at a time so I didn't really notice. She sold drugs. If you use drugs, you sell them too. It is part of the drug culture, per my daughter. If your son told her he doesn't sell drugs, she would not believe him. They sell to get, so to speak. I can not vouch that this is a fact, but she swears it is so take what you want from it.
By the way, pot isn't straight pot anymore. It's far more dangerous than our generation's pot was.
With all the problems your son has had, my guess is that his disappearances are for drug use without your seeing how wasted he is. They find ways to hide it and can be very clever. My daughter's biggest tactic was to use the more dangerous drugs after we fell asleep. She would sneak kids into her room from her window too. We had no idea how bad it was with her so we did go to sleep. We did not know she'd often run the streets after we fell asleep by climbing out her window either until a cop brought her home for curfew. Then we boarded up her window. She still used drugs.
You can sort of tell how your kids are doing by their friends. My daughter never gave up her scumbag friends, most who are still in and out of jail at almost thirty years of age, until she decided to quit. She found it very helpful to move away when she wanted to quit as her old "friends" never left her alone and threatened her, including threatening her life. But she did not make new drug friends when she moved. She was very lonely and spent most of her time indoors or at work (she walked to work and back), but she had decided to quit and get on with her life and preferred being alone to the drug world.
My daughter told me something very interesting just the other day as we got into a discussion about drugs (spurred on by a report on our local schools here). She said that the drug life is really hard and terrible and that she hated it. She said you are always needing to find somebody who will sell to you and then find the money, often without a job, and then they will want you to return the favor and that you had to associate with the scum of the earth. She did not enjoy her drug life and tried several times to quit while living with us, but her buddies would not allow her to quit...they would threaten her family (us) if she didn't find them some speed, for example.
In her case, it was a bullseye when we made her leave the house and she begged her brother in Illinois to let him live with her, even though the rules would be ten times stricter than ours were. Once she left the environment, everything started to turn around. If your child ever seems to want to quit, you may want to find another place for him to live which is far away from the gang he hangsj with now.
At any rate, if your adult child is hanging around with druggies, he is most liekly also using drugs. The turnabout happens when suddenly those friends don't come around anymore and your adult child makes new friends. Druggies don't take kindly to kids who decide to quit. Drugs are their bond. It's what they do. And misery loves company.