As an unlicensed, unbonded, private investigator, I have helped some friends in the past with finding missing brothers or sisters. I have been succesful finding one alive. Two others were confirmed dead. (the mother of one, I couldn't tell her the truth - I lied and said I couldn't find him. I'd rather not see her heartbroken. I just couldn't do it.) About a dozen others I can find any trace of them. By law, if I am not bonded or licensed by a state (almost all states require it), I can't charge for my services.
Simple things you can look for when looking for a person who may be intionally missing
1. Many US states have websites listed under "court records." Washington state, and Maryland, for example. Hardly anyone goes without at least getting a speeding ticket. It would be listed here, including dates, and on occasion, addresses of the offender. Look up the court records in the state and surrounding states from where the person was last seen.
2. State Department of Social and Health Services will tell you whether the person has or is recieiving state-covered medical care, food stamps, etc. It's public record. You need the person's full name and social security number. Again check surrounding states.
3. If you know the name and Social Security Number, and have a major credit card - call one of those free credit score web sites advertised on TV. Pretend you are the missing person looking for your credit score. If the missing person has had any kind of credit and the past 10 or so years, you will learn where from. This is not illegal. Lawyers regularly do this. You can also just apply for a credit rating of the missing person from any one of the three major credit bureaus - pretending to be a landlord renting to said person - and get the same results.
4. Go to the Social Security Death Index on the internet. Type in name and Social Security Number. If the person has died and is not a Jane or John Doe, they may be listed here. Not everyone with a SS number who has died is listed, however. (One of two of my dead brothers, aren't, for example.) Do not pay for this service (ancestory.com, for example). It's free, public information.
5. People likely go back to an area they are familiar with. So if the person grew up in Florida, for example, but lived some place else when they disappeared - it's natural for them to return to Florida, for example.
6. Check the morgue photo section web sites of police and sheriff's offices of some major cities. The Las Vegas Police Department, for example, posts photos of all dead Jane and John does.
7. Some people end up with the only jobs they can get - traveling carnivals that go to county fairs during the summer. Call them. I've never had a problem with any of the companies freely telling me whether or not the person has worked for them, after expalining what I was doing. This is how I found out a mother's son died a good five years before I started looking for him.
If you can't find the person - that doesn't mean they are dead. It's easy to leave the USA or Canada, or stay missing. Remember the Dugan case....the Barefoot Bandit.....for everyone of them, there are probably hundreds of others, for all anyone knows. There are other tips. I'm sure others have their own good ones, too